High strength thin steel sheet having high hydrogen embrittlement resisting property and high workability

ABSTRACT

The present invention provides a high strength thin steel sheet that has high hydrogen embrittlement resisting property and high workability. The high strength thin steel sheet having high hydrogen embrittlement resisting property has a metallurgical structure after stretch forming process to elongate 3% comprises: (i) 1% or more residual austenite; 80% or more in total of bainitic ferrite and martensite; and 9% or less (may be 0%) in total of ferrite and pearlite in terms of proportion of area to the entire structure, wherein the mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher, or (ii) 1% or more residual austenite in terms of proportion of area to the entire structure; mean axis ratio (major axis/minor axis) of the residual austenite grains is 5 or higher; mean length of minor axes of the residual austenite grains is 1 μm or less; minimum distance between the residual austenite grains is 1 μm or less; and the steel has tensile strength of 1180 MPa or higher.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high strength thin steel sheet thathas high hydrogen embrittlement resisting property (particularly thehydrogen embrittlement resisting property after being subjected toforming process) and high workability, especially to a high strengththin steel sheet that has high resistance against fractures due tohydrogen embrittlement such as season crack and delayed fracture thatpose serious problems for steel sheets having tensile strength of 1180MPa or higher, and has high workability.

2. Description of the Related Art

There are increasing demands for the steel sheet, that is pressed orbent into a form of a high-strength component of automobile orindustrial machine, to have both high strength and high ductility at thesame time. In recent years, there are increasing needs for high strengthsteel sheets having strength of 1180 MPa or higher, as the automobilesare being designed with less weight. A type of steel sheet that isregarded as promising to satisfy these needs is TRIP (transformationinduced plasticity) steel sheet.

The TRIP steel sheet includes residual austenite structure and, whenprocessed to deform, undergoes considerable elongation due to inducedtransformation of the residual austenite (residual γ) into martensite bythe action of stress. Known examples of the TRIP steel include TRIP typecomposite-structure steel (TPF steel) that consists of polygonal ferriteas the matrix phase and residual austenite; TRIP type temperedmartensite steel (TAM steel) that consists of tempered martensite as thematrix phase and residual austenite; and TRIP type bainitic steel (TBFsteel) that consists of bainitic ferrite as the matrix phase andresidual austenite. Among these, the TBF steel has long been known(described, for example, in NISSIN STEEL TECHNICAL REPORT, No. 43,December 1980, pp 1-10), and has such advantages as the capability toreadily provide high strength due to the hard bainitic ferritestructure, and the capability to show outstanding elongation becausefine residual austenite grains can be easily formed in the boundary oflath-shaped bainitic ferrite in the bainitic ferrite structure. The TBFsteel also has such an advantage related to manufacturing, that it canbe easily manufactured by a single heat treatment process (continuousannealing process or plating process).

In the realm of high strength of 1180 MPa upward, however, the TRIPsteel sheet is known to suffer a newly emerging problem of delayedfracture caused by hydrogen embrittlement, similarly to the conventionalhigh strength steel. Delayed fracture refers to the failure ofhigh-strength steel under stress, that occurs as hydrogen originating incorrosive environment or the atmosphere infiltrates and diffuses inmicrostructural defects such as dislocation, void and grain boundary,and makes the steel brittle. This results in decreases in ductility andtoughness of the metallic material.

It has been well known that the high strength steel that is widely usedin the manufacture of PC steel wire and line pipe experiences hydrogenembrittlement (pickling embrittlement, plating embrittlement, delayedfracture, etc.) caused by the infiltration of hydrogen into the steelwhen tensile strength of the steel becomes 980 MPa or higher.Accordingly, most of technologies of improving hydrogen embrittlementresisting property have been developed aiming at steel members such asbolt. “New Development in Elucidation of Delayed Fracture” (published byThe Iron and Steel Institute of Japan in January, 1997), for example,describes that it is effective in improving the resistance againstdelayed fracture to add element such as Cr, Mo or V that demonstratesresistance against temper softening to the metal structure that is basedon tempered martensite as the major phase. This technology is intendedto cause the delayed fracture to take place within grains instead of inthe grain boundaries, thereby to constrain the fracture from occurring,by precipitating alloy carbide and making use thereof as the site fortrapping hydrogen.

Thin steel sheets having strength higher than 780 MPa have rarely beenused for the reason of workability and weldability. Also hydrogenembrittlement has rarely been regarded as a problem for thin steelsheets where hydrogen that has infiltrated therein is immediatelyreleased due to the small thickness. For these reasons, much effortshave not been dedicated to counter the hydrogen embrittlement. In recentyears, however, higher strength is required of the reinforcement memberssuch as bumper, impact beam and seat rail, etc., in order to meet therequirement of weight reduction of the automobile and to improve thecollision safety. Automobile components that are shaped by pressing orbending process such as pillar are also required to have higherstrength. As a result, there have been increasing demands for highstrength steel sheet having strength of 980 MPa or higher for themanufacture of these parts. This makes it necessary to improve hydrogenembrittlement resisting property of the high strength steel sheet.

Use of the technology addressed to the bolt steel described above may beconsidered for improving the hydrogen embrittlement resisting propertyof the high strength steel sheet. However, in the case of “NewDevelopment in Elucidation of Delayed Fracture” (published by The Ironand Steel Institute of Japan in January, 1997), for example, 0.4% orhigher of C content and much alloy elements are contained, and thereforeapplication of this technology to a thin steel sheet compromises theworkability required of the thin steel sheet. The technology also has adrawback related to the manufacturing process, since it takes severalhours or longer period of heat treatment to cause the alloy carbide toprecipitate. Therefore, improvement of the hydrogen embrittlementresisting property of a thin steel sheet requires it to develop a noveltechnology.

It is relatively easy to achieve a high strength with quench-hardened(tempered) martensite steel that has been commonly used as ahigh-strength steel. However, improvement of the workability withoutvariability essentially requires it to provide a tempering process whichmakes it necessary to strictly control the temperature and duration ofthe process. This also sometimes increases the possibility of temperingembrittlement and makes it difficult to reliably improve workability.Although there is a steel of composite structure of martensite andferrite or the like developed to improve ductility, such a steel has ahigh notch sensitivity due to mixed presence of hard phase and softphase, thus making it difficult to achieve sufficient improvement ofhydrogen embrittlement resisting property.

Hydrogen-induced delayed fracture is believed to occur in such a steelthat contains martensite, because hydrogen is concentrated in grainboundaries of prior austenite thereby to form voids or other defectsthat become the starting points of the fracture. Common practice thathas been employed to decrease the sensitivity to delayed fracture is todiffuse fine grains of carbide or the like uniformly as the site fortrapping hydrogen, thereby to decrease the concentration of diffusivehydrogen. However, even when a large number of carbide grains or thelike are diffused as the trap site for hydrogen, there is a limitationto the hydrogen trapping capability and delayed fracture attributable tohydrogen cannot be fully suppressed.

Japanese Unexamined Patent Publication (Kokai) No. 11-293383 describes atechnology to improve the hydrogen embrittlement resisting property ofsteel sheet, where hydrogen-induced defects can be suppressed by havingoxides that include Ti and Mg exist as the main components in thestructure. However, this technology is intended for thick steel sheetsand, although consideration is given to delayed fracture after weldingwith a large input heat, no consideration is given to the environment(for example, corrosive environment, etc.) in which automobile partsmanufactured by using thin steel sheets are used.

Japanese Unexamined Patent Publication (Kokai) No. 2003-166035 describesthat it is made possible to improve the ductility and delayed fractureresistance after being subjected to forming process, by controlling themutual relationships between 1) the form (standard deviation and meangrain size) in which oxide, sulfide, composite crystallization productor composite precipitate of Mg is dispersed, 2) volumetric proportion ofresidual austenite and 3) strength of the steel sheet. However, it isdifficult to improve the hydrogen embrittlement resisting property insuch an environment as hydrogen is generated through corrosion of thesteel sheet simply through the trapping effect achieved by controllingthe form of precipitate.

It has been a common practice in the past to reduce the residualaustenite that was believed to have an adverse effect on the hydrogenembrittlement resisting property. In recent years, however, the effectof residual austenite on the improvement of hydrogen embrittlementresisting property has been recognized and accordingly much attentionhas been paid to the TRIP steel that contains residual austenite.

Tomohiko HOJO et. al “Hydrogen Embrittlement of High Strength Low AlloyTRIP Steel (Part 1: Hydrogen Absorbing Characteristic and Ductility”,The Society of Materials Science, Japan, proceedings of 51^(st) academiclecture meeting, 2002, vol. 8, pp 17-18 and Tomohiko HOJO et. al“Influence of Austempering Temperature on Hydrogen Embrittlement ofHigh, for example, describe investigations into the hydrogenembrittlement resisting property of the TRIP steel. It is pointed outthat, among the TRIP steels, TBF steel has particularly high hydrogenabsorbing capacity, and observation of a fracture surface of the TBFsteel shows the restriction of quasi cleavage fracture due to storage ofhydrogen. However, the TBF steels reported in the documents describedabove show delayed fracture characteristic of about 1000 seconds at themost in terms of the time before crack occurrence measured in cathodecharging test, indicating that these steels are not meant to endure theharsh operating environment such as that of automobile parts over a longperiod of time. Moreover, since the heat treatment conditions reportedin the documents described above involve heating temperature being sethigher, there are such problems as low efficiency of practicalmanufacturing process. Thus it is strongly required to develop a newspecies of TBF steel that provides high production efficiency as well.Also there has been such a problem that press forming operation leads tolower hydrogen embrittlement resisting property.

As described above, there have been virtually no TRIP steels containingresidual austenite that have been developed so as to demonstrate highworkability when processed to form parts, by taking measures to counterhydrogen embrittlement after the forming process in consideration of theharsh operating environment such as that of automobile parts over a longperiod of time.

SUMMARY OF THE INVENTION

The present invention has been made with the background described above,and has an object of providing a high strength thin steel sheet thatshows high hydrogen embrittlement resisting property in a harshoperating environment over a long period of time after the process offorming the steel sheet into a part, and has improved workability andtensile strength of 1180 MPa or higher.

In order to achieve the object described above, the present inventorsconducted a research on a steel sheet that shows high hydrogenembrittlement resisting property after the forming process, anddemonstrates improved workability which is the characteristic propertyof the TRIP steel sheet during the forming process. Through theresearch, it was found that it is very important to control themetallurgical structure after the forming process in order to achievehigh hydrogen embrittlement resisting property after the formingprocess. Specifically, it was found that it is important that the metalstructure after the stretch forming process is constituted from:

1% or more of residual austenite;

80% or more in total of bainitic ferrite and martensite; and

9% or less (may be 0%) in total of ferrite and pearlite in terms of theproportion of area to the entire structure, wherein the mean axis ratio(major axis/minor axis) of the residual austenite grains is 5 or higher.

A first high strength thin steel sheet having high hydrogenembrittlement resisting property according to the present inventioncomprises higher than 0.25 and up to 0.60% of C (contents of componentsgiven in terms of percentage in this patent application all refer topercentage by weight), 1.0 to 3.0% of Si, 1.0 to 3.5% of Mn, 0.15% orless P, 0.02% or less S and 1.5% or less (higher than 0%) of Al, whileiron and inevitable impurities making up the rest, wherein themetallurgical structure comprises:

1% or more residual austenite;

80% or more in total of bainitic ferrite and martensite; and

9% or less (may be 0%) in total of ferrite and pearlite in theproportion of area to the entire structure, and wherein the mean axisratio (major axis/minor axis) of the residual austenite grains is 5 orhigher, and the steel has tensile strength of 1180 MPa or higher.

The present inventors also conducted another research from a point ofview that was different from that of the former research, and found thathigh hydrogen embrittlement resisting property after the forming processcan be achieved by controlling the metal structure after the formingprocess as follows. It is important that the metal structure after theforming process comprises:

1% or more residual austenite;

the mean axis ratio (major axis/minor axis) of the residual austenitegrains is 5 or higher.

mean length of minor axes of the residual austenite grains is 1 μm orless; and minimum distance between the residual austenite grains is 1 μmor less.

When the metal structure is controlled as described above, hydrogenembrittlement resisting property of the high strength thin steel sheetcan be sufficiently improved without adding much alloy elements. Thephrase “after the forming process” means the state of the steel sheetafter being stretched with an elongation ratio of 3%. Specifically, thesteel sheet is subjected to uniaxial stretching of 3% at the roomtemperature (the stretching process of 3% elongation may hereinafter bereferred to simply as “processing”).

A second high strength thin steel sheet having high hydrogenembrittlement resisting property according to the present inventioncomprises higher than 0.25 and up to 0.60% of C, 1.0 to 3.0% of Si, 1.0to 3.5% of Mn, 0.15% or less P, 0.02% or less S, 0.5% or less (higherthan 0%) Al, while iron and inevitable impurities making up the rest,wherein the metal structure after the stretch forming process of 3%elongation comprises:

1% or more residual austenite;

the mean axis ratio (major axis/minor axis) of the residual austenitegrains is 5 or higher;

mean length of minor axes of the residual austenite grains is 1 μm orless;

minimum distance between the residual austenite grains is 1 μm or less;and

tensile strength is 1180 MPa or higher.

According to the present invention, it is made possible to manufacture,with a high level of productivity, a high strength thin steel sheethaving tensile strength of 1180 MPa or higher that neutralizes hydrogenthat infiltrates from the outside after the steel sheet has been formedinto a part thereby to maintain satisfactory hydrogen embrittlementresisting property, and demonstrates high workability during the formingprocess. Use of the high strength thin steel sheet makes it possible tomanufacture high strength parts that hardly experience delayed fracture,such as bumper, impact beam and other reinforcement members and otherautomobile parts such as seat rail, pillar, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a part used in pressurecollapse test in Example 1.

FIG. 2 is a side view schematically showing the setup of pressurecollapse test in Example 1.

FIG. 3 is a schematic perspective view of a part used in impactresistance test in Example 1.

FIG. 4 is a sectional view along A-A in FIG. 3.

FIG. 5 is a side view schematically showing the setup of impactresistance test in Example 1.

FIG. 6 is a photograph of TEM observation (magnification factor 15000)of No. 101 (inventive steel) of Example 1.

FIG. 7 is a photograph of TEM observation (magnification factor 15000)of No. 120 (comparative steel) of Example 1.

FIG. 8 is a photograph of TEM observation (magnification factor 15000)of No. 201 (inventive steel) of Example 2.

FIG. 9 is a photograph of TEM observation (magnification factor 15000)of No. 220 (comparative steel) of Example 2.

FIG. 10 is a graph showing the relationship between the mean axis ratioof the residual austenite grains and hydrogen embrittlement risk index.

FIG. 11 is a diagram schematically showing the minimum distance betweenresidual austenite grains.

FIG. 12 is a photograph of TEM observation (magnification factor 15000)of No. 301 (inventive steel) of Example 3.

FIG. 13 is a photograph of TEM observation (magnification factor 60000)of No. 301 (inventive steel) of Example 3.

FIG. 14 is a photograph of TEM observation (magnification factor 15000)of No. 313 (comparative steel) of Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

The first high strength thin steel sheet according to the presentinvention is constituted from higher than 0.25 and up to 0.60% of C(contents of components given in terms of percentage in this patentapplication all refer to percentage by weight), 1.0 to 3.0% of Si, 1.0to 3.5% of Mn, 0.15% or less P, 0.02% or less S, 1.5% or less (higherthan 0%) of Al, 1.0% or less (higher than 0%) of Mo and 0.1% or less(higher than 0%) of Nb, while iron and inevitable impurities making upthe rest, and is characterized in that:

(i) the metal structure after the forming process contains: 1% or moreresidual austenite;

80% or more in total of bainitic ferrite and martensite; and

9% or less (may be 0%) in total of ferrite and pearlite in terms of theproportion of area to the entire structure, and

the mean axis ratio (major axis/minor axis) of the residual austenitegrains is 5 or higher; and

(ii) the steel contains a specified amount of Mo and/or Nb.

The requirements described above have reasons as follows.

(Metal Structure after Stretch Forming by 3% Elongation)

Metal structure after stretch forming process by 3% elongation wasspecified because, in various experiments conducted for the actualprocessing conditions in the manufacture of a part, best correlationbetween the result of laboratory test and the actual occurrence ofcracks in the part was observed when the part was processed by stretchforming with an elongation ratio of 3%.

The phrase “after the forming process” means the state of the steelsheet after being stretch formed with elongation of 3%. Specifically,the steel sheet is subjected to elongation of 3% by uniaxial stretchingat the room temperature (the stretch forming process of 3% elongationmay hereinafter be referred to simply as “process”).

(1% or More Residual Austenite in the Area Proportion to the EntireStructure)

It is necessary that the metal contains 1% or more residual austenite inthe area proportion to the entire structure after the process of formingthe part, in order to achieve high hydrogen embrittlement resistingproperty in harsh operating environment over an extended period of timeafter forming the part. Content of the residual austenite is preferably2% or higher, and more preferably 3% or higher. Since the desired levelof high strength cannot be obtained when an excessive amount of residualaustenite is contained after processing, it is recommended to set anupper limit of 20% (more preferably 15%) to the residual austenitecontent.

(Mean Axis Ratio (Major Axis/Minor Axis) of the Residual AusteniteGrains: 5 or Higher)

Lath-shaped grains of residual austenite after the process have farhigher capacity of trapping hydrogen than carbide. When the mean axisratio (major axis/minor axis) of the residual austenite grains is 5 orhigher, in particular, it was found that hydrogen that infiltrates fromthe outside through atmospheric corrosion can be substantiallyneutralized thereby to achieve remarkable achievement in hydrogenembrittlement resisting property. The mean axis ratio of the residualaustenite grains is preferably 10 or higher, and more preferably 15 orhigher.

The residual austenite refers to a region that is observed as FCC (facecentered cubic lattice) by the FE-SEM/EBSP method which will bedescribed later. Measurement by the EBSP may be done, for example, bymeasuring a measurement area (about 50 by 50 μm) at an arbitrarilychosen position in a surface parallel to the rolled surface at aposition of one quarter of the thickness at measuring intervals of 0.1μm. The measuring surface is prepared by electrolytic polishing in orderto prevent the residual austenite from transforming. Then the test pieceis set in the lens barrel of an FE-SEM equipped with an EBSP detector(of which details will be described later) and is irradiated withelectron beam. An EBSP image projected onto a screen is captured by ahigh sensitivity camera (VE-1000-SIT manufactured by Dage-MTI Inc.) andis sent to a computer. The computer carries out image analysis andgenerates color mapping of the FCC phase through comparison with astructural pattern simulated with a known crystal system (FCC (facecentered cubic lattice) phase in the case of residual austenite). Areaproportion of the region that is mapped as described above is taken asthe area proportion of the residual austenite. This analysis was carriedout by means of hardware and software of OIM (Orientation ImagingMicroscopy™) system of TexSEM Laboratories Inc.

The mean axis ratio was determined by measuring the major axis and minoraxis of residual austenite crystal grains existing in each of threearbitrarily chosen fields of view in the observation by means of TEM(transmission electron microscope) with magnification factor of 15000,and averaging the ratios of major axis to minor axis.

(80% or More in Total of Bainitic Ferrite and Martensite)

In order to decrease the number of intergranular fracture initiatingpoints in the steel thereby to surely decrease the concentration ofdiffusive hydrogen to a harmless level and achieve a high strength, itis desirable to form the matrix phase of the steel structure afterprocessing from a binary phase structure of bainitic ferrite andmartensite with the bainitic ferrite acting as the main phase, insteadof the single phase structure of martensite that is generally used forhigh strength steels.

In the single phase structure of martensite, a carbide (for example,film-like cementite) is likely to precipitate in the grain boundaries,thus making intergranular fracture likely to occur. In the case of thebinary structure of bainitic ferrite and martensite with the bainiticferrite acting as the main phase, in contrast, the bainitic ferrite is ahard phase and therefore it is easy to increase the strength of theentire structure as in the case of the single phase of martensite. Thehydrogen embrittlement resisting property can also be improved as muchhydrogen is trapped in the dislocations. It also has such an advantagethat coexistence of the bainitic ferrite and the residual austenitewhich will be described later prevents the generation of carbide thatacts as the intergranular fracture initiating points, and it becomeseasier to create the lath-shaped residual austenite in the boundaries oflath-shaped bainitic ferrite.

Accordingly, it is required in the present invention that the binarystructure of bainitic ferrite and martensite occupy 80% or more,preferably 85% or more and more preferably 90% or more of the entirestructure after the stretch forming processing to elongate by 3%. Upperlimit of the proportion may be determined by the balance with otherstructure (residual austenite), and is set to 99% when the otherstructures (ferrite, etc.) than the residual austenite is not contained.

The bainitic ferrite referred to in the present invention isplate-shaped ferrite having a lower structure of high density ofdislocations. It is clearly distinguished from polygonal ferrite thathas lower structure including no or very low density of dislocations, bySEM observation as follows.

Area proportion of bainitic ferrite structure is determined as follows.A test piece is etched with Nital etchant. A measurement area (about 50by 50 μm) at an arbitrarily chosen position in a surface parallel to therolled surface at a position of one quarter of the thickness is observedwith SEM (scanning electron microscope) (magnification factor of 1500)thereby to determine the area proportion.

Bainitic ferrite is shown with dark gray color in SEM photograph(bainitic ferrite, residual austenite and martensite may not bedistinguishable in the case of SEM observation), while polygonal ferriteis shown black in SEM photograph and has polygonal shape that does notinclude residual austenite and martensite inside thereof.

The SEM used in the present invention is a high-resolution FE-SEM (FieldEmission type Scanning Electron Microscope XL30S-FEG manufactured byPhilips Inc.) equipped with an EBSP (Electron Back Scattering Pattern)detector, that has a merit of being capable of analyzing the areaobserved by the SEM at the same time by means of the EBSP detector. EBSPdetection is carried out as follows. When the sample surface isirradiated with electron beam, the EBSP detector analyzes the Kikuchipattern obtained from the reflected electrons, thereby to determine thecrystal orientation at the point where the electron beam has hit upon.Distribution of orientations over the sample surface can be measured byscanning the electron beam two-dimensionally over the sample surfacewhile measuring the crystal orientation at predetermined intervals. TheEBSP detection method has such an advantage that different structuresthat are regarded as the same structure in the ordinary microscopicobservation but have different crystal orientations can be distinguishedby the difference in color tone.

(9% or Less (may be 0%) in Total of Ferrite and Pearlite)

The steel sheet after the processing may be constituted either from onlythe structures described above (namely, a mixed structure of bainiticferrite+martensite and residual austenite), or may include otherstructure such as ferrite (the term ferrite used herein refers topolygonal ferrite, that is a ferrite structure that includes no or veryfew dislocations) or pearlite to such an extent that the effect of thepresent invention is not compromised. Such additional components arestructures that can inevitably remain in the manufacturing process ofthe present invention, of which concentration is preferably as low aspossible, within 9%, preferably less than 5% and more preferably lessthan 3% according to the present invention.

In order to maintain high hydrogen embrittlement resisting propertyafter the forming process, for example, large content of residualaustenite of 5% or more may be contained in the steel sheet prior to theforming process, or large amount of fine residual austenite grains maybe dispersed in the structure. Alternatively, forming process conditionsmay be controlled so as to make the residual austenite less likely totransform (for example, form the part by bending operation or controlthe forming temperature and/or stretching speed). The most desirablemeans of improving the workability and hydrogen embrittlement resistingproperty at the same time while maintaining the content of residualaustenite before and after the processing substantially constant withinan appropriate range and maintaining other properties (high strength,etc.) is to satisfy the following requirements (A) and (B).

(A) Increase C content in the composition and increase the concentrationof C in the residual austenite.

Although residual austenite transforms into martensite when the steelsheet is deformed (processed), high content of C in the residualaustenite stabilizes it so that further transformation becomes unlikelyto occur. Thus residual austenite can be retained after the formingprocess, thereby maintaining the high hydrogen embrittlement resistingproperty.

According to the present invention, higher than 0.25% of C is containedin order to achieve the effects described above. C is also an elementrequired to achieve a high strength of 1180 MPa or higher, and 0.27% ormore, preferably 0.30% or more C is contained. However, in order toensure corrosion resistance, concentration of C is limited within 0.6%,preferably 0.55% or lower and more preferably 0.50% or lower in thepresent invention.

It is recommended to increase the C content in the steel sheet asdescribed above, thereby to maintain the concentration of C in theresidual austenite (CγR) of 0.8% or higher. Controlling the value of CγRto 0.8% or higher enables it to effectively improve the elongationproperty, which is preferably 1.0% or higher and more preferably 1.2% orhigher. While it is preferable that CγR is as high as possible, it isconsidered that in practice there is an upper limit of around 1.6%.

(B) Form the residual austenite in fine lath-shaped grains.

Residual austenite formed in fine lath-shaped grains does not undergoexcessive transformation during the forming process, thus enabling it tomaintain the residual austenite.

Some of the TRIP steels of the prior art have unsatisfactory hydrogenembrittlement resisting property despite sufficient content of residualaustenite. The reason may be that, since residual austenite existing inthe TRIP steel of the prior art generally has block shape of size onmicrometer order, it can easily transform into martensite when beingstressed and may act as the starting point of mechanical destruction.Through a research conducted by the present inventors, it was found thatresidual austenite formed in lath shape is more stable and less likelyto transform into martensite than the residual austenite of the priorart that has block shape, given the same amount of deformation. Thisdifference may be caused by the difference in the way in which thestress is applied and in the difference in spatial restriction, althoughnot fully elucidated. Stabilization of residual austenite duringprocessing has no influence on the lowering of workability of TRIP steelsheet due to induced transformation. According to the present invention,induced transformation proceeds efficiently and high workability can beachieved without hardly reducing the residual austenite, when theresidual austenite is formed into fine lath shape as described above.

Lath-shaped grains of residual austenite having mean axis ratio (majoraxis/minor axis) of 5 or higher (preferably 10 or higher, and morepreferably 15 or higher) minimizes the decrease of residual austeniteduring processing and makes it possible to easily achieve mean axisratio (major axis/minor axis) of 5 or higher after processing, put thehydrogen absorbing capability of the residual austenite into full playand greatly improve hydrogen embrittlement resisting property. While noupper limit of the mean axis ratio is specified for the consideration ofimprovement in hydrogen embrittlement resisting property, the residualaustenite grains are required to have certain level of thickness inorder to achieve the TRIP effect during processing. Thus it ispreferable to set an upper limit to 30, more preferably to 20 or less.

According to a preferred embodiment of the present invention, Mo and Nbare added for the purpose of reducing the size of the residual austenitegrains. Mo has the effects of strengthening the grain boundary so as tosuppress hydrogen embrittlement from occurring, in addition to reducingthe size of the residual austenite grains. Mo also has the effect ofimproving the hardenability of the steel sheet. It is recommended to add0.005% or more of Mo in order to achieve these effects. More preferably0.1% or more of Mo is added. However, since the effects described abovereach saturation when the Mo content exceeds 1.0%, resulting ineconomical disadvantage, Mo content is limited to 0.8% or less and morepreferably to 0.5% or less.

Nb, in cooperation with Mo, acts very effectively to decrease the grainsize of the structure. Nb also has the effect of increasing the strengthof the steel sheet. It is recommended to add 0.005% or more of Nb inorder to achieve these effects. More preferably 0.01% or more of Nb isadded. However, since the effects described above reach saturation whenan excessive Nb content is included, resulting in economicaldisadvantage, Nb content is limited to 0.1% or less and more preferablyto 0.08% or less.

In order to readily obtain the structure described above afterprocessing, it is recommended to make the steel sheet constituted from80% or more (preferably 85% or more, and more preferably 90% or more) intotal of bainitic ferrite and martensite, and 9% or less (preferablyless than 5%, and more preferably less than 3% containing 0%) in totalof ferrite and pearlite making up the rest of the residual austenitebefore processing. This is because it is preferable that the steel sheethas high hydrogen embrittlement resisting property prior to theprocessing as well as after the processing, and this constitution makesit easier to achieve the specified strength.

While this embodiment is characterized in that metal structure iscontrolled after processing, it is necessary to control the othercomponents as described below, in order to form the metal structure andefficiently improve hydrogen embrittlement resisting property andstrength thereby to ensure ductility required for the thin steel sheet.

<Si: 1.0 to 3.0%>

Si is an important element that effectively suppresses the residualaustenite from decomposing and carbide from being generated, and is alsoeffective in enhancing substitution solid solution for hardening thematerial. In order to make full use of these effects, it is necessary toinclude Si in a concentration of 1.0% or higher, preferably 1.2% orhigher and more preferably 1.5% or higher. However, excessively highcontent of Si leads to conspicuous formation of scales due to hotrolling and makes it necessary to remove flaws, thus adding up to themanufacturing cost and resulting in economical disadvantage. ThereforeSi content is controlled within 3.0%, preferably within 2.5% and morepreferably within 2.0%.

<Mn: 1.0 to 3.5%>

Mn is an element required to stabilize austenite and obtain desiredresidual austenite. In order to make full use of this effect, it isnecessary to add Mn in concentration of 1.0% or higher, preferably 1.2%or higher, and more preferably 1.5% or higher. However, adding anexcessive amount Mn leads to conspicuous segregation and poorworkability. Therefore upper limit to the concentration of Mn is set to3.5% and more preferably to 3.0% or less.

<P: 0.15% or Lower (Higher than 0%)>

P intensifies intergranular fracture due to intergranular segregation,and the content thereof is therefore preferably as low as possible.Upper limit to the concentration of P is set to 0.15%, preferably 0.1%or less and more preferably to 0.05% or less.

<S: 0.02% or Lower (Higher than 0%)>

S intensifies the absorption of hydrogen into the steel sheet incorrosive environment, and the content thereof is therefore preferablyas low as possible. Upper limit to the concentration of S is set to0.02%.

<Al: 1.5% or Less (Higher than 0%)> (In the Case of Inventive Steel 1)

<Al: 0.5% or Less (Higher than 0%)> (In the Case of Inventive Steel 2)

0.01% or higher content of Al may be included for the purpose ofdeoxidation. In addition to deoxidation, Al also has the effects ofimproving the corrosion resistance and improving hydrogen embrittlementresisting property.

The mechanism of improving the corrosion resistance is supposedly basedon the improvement of corrosion resistance of the matrix phase per seand the effect of formation rust generated by atmospheric corrosion,while the effect of the formation rust presumably has greatercontribution. This is supposedly because the formation rust is denserand better in protective capability than ordinary iron rust, andtherefore depresses the progress of atmospheric corrosion so as todecrease the amount of hydrogen generated by the atmospheric corrosion,thereby to effectively suppress the occurrence of hydrogenembrittlement, and hence the delayed fracture.

While details of the mechanism of improvement of the hydrogenembrittlement resistance by Al is not known, it is supposed thatcondensing of Al on the surface of the steel makes it difficult forhydrogen to infiltrate into the steel, and the decreasing diffusion rateof hydrogen in the steel makes it difficult for hydrogen to migrate sothat hydrogen embrittlement becomes less likely to occur. In addition,stability of lath-shaped residual austenite improved by the addition ofAl is believed to contribute to the improvement of hydrogenembrittlement resisting property.

In order to effectively achieve the effects of Al in improving thecorrosion resistance and improving the hydrogen embrittlement resistingproperty, Al content is controlled to 0.2% or higher, preferably 0.5% orhigher.

However, Al content must be controlled within 1.5% in order to preventinclusions such as alumina from increasing in number and size so as toensure satisfactory workability, ensure the generation of fine residualaustenite grains, suppress corrosion from proceeding from the inclusioncontaining Al as the starting point, and prevent the manufacturing costfrom increasing. In view of the manufacturing process, it is preferableto control so that A3 point is not higher than 1000° C.

As the Al content increases, inclusions such as alumina increase andworkability becomes poorer. In order to suppress the generation of theinclusions such as alumina and make a steel sheet having higherworkability, Al content is restricted within 0.5%, preferably within0.3% and more preferably within 0.1%.

While constituent elements (C, Si, Mn, P, S, Al, Mo, Nb) of the steel ofthis embodiment is as described above with the rest substantially beingFe, it may include inevitable impurities introduced into the steeldepending on the stock material, production material, manufacturingfacility and other circumstances, containing 0.001% or less of N(nitrogen) In addition, other elements as described below may beintentionally added to such an extent that does not adversely affect theeffects of the present invention.

<B: 0.0002 to 0.01%>

B is effective in increasing the strength of the steel sheet, and it ispreferable that 0.0002% or more (more preferably 0.0005% or more) B iscontained. However, an excessive content of B leads to poor hotprocessing property. Therefore, it is preferable to control theconcentration of B to within 0.01% (more preferably within 0.005%).

<At Least One Selected from Among Ca: 0.0005% to 0.005%, Mg: 0.0005% to0.01% and REM: 0.0005% to 0.01%)

Ca, Mg and REM (rare earth element) are effective in suppressing anincrease in hydrogen ion concentration, that is, a decrease in pH in theatmosphere of the interface due to corrosion of the steel sheet surface,thereby to improve the corrosion resistance of the steel sheet. Theseelements are also effective in controlling the form of sulfide containedin the steel and improve the workability of the steel. In order toachieve the effects described above, it is recommended to add each ofCa, Mg and REM in concentration of 0.0005% or higher. However, sinceexcessive contents of these elements leads to poor workability, it ispreferable to keep the concentration of Ca to 0.005% or less andconcentration of Mg and REM each within 0.01%.

Second Embodiment

The second high strength thin steel sheet according to the presentinvention is constituted from higher than 0.25% and up to 0.60% of C(contents of components given in terms of percentage in this patentapplication all refer to percentage by weight), 1.0 to 3.0% of Si, 1.0to 3.5% of Mn, 0.15% or less of P, 0.02% or less of S, 1.5% or less(higher than 0%) of Al while iron and inevitable impurities constitutethe rest, wherein: (i) the structure after the forming processcomprises:

1% or more residual austenite;

mean axis ratio (major axis/minor axis) of the residual austenite grainsis 5 or higher;

80% or more in total of bainitic ferrite and martensite; and

9% or less (may be 0%) in total of ferrite and pearlite in theproportion of area to the entire structure, and

(ii) the steel contains specified amount of Cu and/or Ni.

The requirements (i) have the reasons as described above.

The requirement (ii) described above has the reason as follows.

Specific measures were studied to retain residual austenite afterprocessing, control the shape of the residual austenite grains, improvethe hydrogen trapping capability and reliably reduce the concentrationof diffusive hydrogen in the steel sheet to a harmless level by: (a)sufficiently suppressing the generation of hydrogen from the steel sheetin corrosive environment; and (b) suppressing hydrogen that has beengenerated from infiltrating the steel sheet.

It was found that it is very effective to include 0.003 to 0.5% of Cuand/or 0.003 to 1.0% of Ni in achieving the objectives of (a) and (b),and that the effect of improving hydrogen embrittlement resistingproperty through control of the structure can be achieved further bycontaining these elements.

Specifically, presence of Cu and Ni improves the corrosion resistance ofthe steel, and effectively suppresses the generation of hydrogen due tocorrosion of the steel sheet. These elements also have the effect ofpromoting the generation of iron oxide, α-FeOOH, that is believed to beparticularly stable thermodynamically and have protective property amongvarious forms of rust generated in the atmosphere. By assisting thegeneration of this rust, it is made possible to suppress hydrogen thathas been generated from infiltrating into the spring steel thereby tosufficiently improve the hydrogen embrittlement resisting property toendure in harsh corrosive environment. This effect can be achievedparticularly satisfactorily when Cu and Ni are contained at the sametime.

In order to achieve the effects described above, concentration of Cu, ifadded, should be 0.003% or higher, preferably 0.05% or higher and morepreferably 0.1% or higher. Concentration of Ni, if added, should be0.003% or higher, preferably 0.05% or higher and more preferably 0.1% orhigher.

Since excessively high concentration of either Cu or Ni is detrimentalto workability, it is preferable to limit the Cu content to 0.5% orlower and limit the Ni content to 1.0% or lower.

In order to achieve high hydrogen embrittlement resisting property afterthe forming process by retaining the predetermined amount of residualaustenite after the forming process as in (i) described above, forexample, 5% or more residual austenite may be contained in the steelsheet prior to the forming process, or large amount of fine residualaustenite grains may be dispersed in the structure. Alternatively,forming process conditions may be controlled so as to make the residualaustenite less likely to transform (for example, form the part bybending operation or control the forming temperature and/or stretchingspeed). The most desirable means of improving the workability andhydrogen embrittlement resisting property at the same time whilemaintaining the content of residual austenite before and after theprocessing substantially constant within an appropriate range andmaintaining other properties (high strength, etc.) is to satisfy therequirements (A) and (B) described previously.

While this embodiment is characterized in that metal structure iscontrolled after processing and predetermined amount of Cu and/or Ni areadded, it is necessary to control the other components as describedbelow, in order to readily form the metal structure and efficientlyimprove hydrogen embrittlement resisting property and strength therebyto ensure ductility required for the thin steel sheet.

While constituent elements (C, Si, Mn, P, S, Al, Cu and/or Ni) of thesteel of this embodiment are as described above with the restsubstantially being Fe, it may include inevitable impurities introducedinto the steel depending on the stock material, production material,manufacturing facility and other circumstances, containing 0.001% orless of N (nitrogen). In addition, other elements as described below maybe intentionally added to such an extent that does not adversely affectthe effects of the present invention.

<Ti and/or V: 0.003 to 1.0% in total>

Ti has the effect of assisting in the generation of protective rust,similarly to Cu and Ni. The protective rust has a very valuable effectof suppressing the generation of β-FeOOH that appears in chlorideenvironment and has adverse effect on the corrosion resistance (andhence on the hydrogen embrittlement resisting property). Formation ofsuch a protective rust is promoted particularly by adding Ti and V (orZr). Ti renders the steel high corrosion resistance, and also has theeffect of cleaning the steel.

V is effective in increasing the strength of the steel sheet anddecreasing the size of crystal grains, in addition to having the effectof improving hydrogen embrittlement resistance through cooperation withTi, as described previously.

In order to fully achieve the effect of Ti and/or V described above, itis preferable to add Ti and/or V in total concentration of 0.003% orhigher (more preferably 0.01% or higher). For the purpose of improvinghydrogen embrittlement resisting property, in particular, it ispreferable to add more than 0.03% of Ti, more preferably 0.05% or moreof Ti. However, the effects described above reach saturation when anexcessive amount of Ti is added, resulting in economical disadvantage.Excessive V content also increases the precipitation of muchcarbonitride and leads to poor workability and lower hydrogenembrittlement resisting property. Therefore, it is preferable to controlthe total concentration of Ti and/or V to within 1.0%, more preferablywithin 0.5%.

<Zr: 0.003 to 1.0%>

Zr is effective in increasing the strength of the steel sheet anddecreasing the crystal grain size, and also has the effect of improvinghydrogen embrittlement resisting property through cooperation with Ti.In order to sufficiently achieve these effects, it is preferable that0.003% or more of Zr is contained. However, excessive Zr contentincreases the precipitation of carbonitride and leads to poorworkability and lower hydrogen embrittlement resisting property.Therefore, it is preferable to control the concentration of Zr to within1.0%.

<Mo: 1.0% or Less (Higher than 0%)>

Mo has the effects of stabilizing austenite so as to retain the residualaustenite, and suppress the infiltration of hydrogen thereby to improvehydrogen embrittlement resisting property. Mo also has the effect ofimproving the hardenability of the steel sheet. In addition, Mostrengthens the grain boundary so as to suppress hydrogen embrittlementfrom occurring. It is recommended to add 0.005% or more Mo in order toachieve these effects. More preferably 0.1% or more Mo is added.However, since the effects described above reach saturation when the Mocontent exceeds 1.0%, resulting in economical disadvantage, Mo contentis limited to 0.8% or less and more preferably to 0.5% or less.

<Nb: 0.1% or Less (Higher than 0%)>

Nb is very effective in increasing the strength of the steel sheet anddecreasing the grain size of the structure. Nb achieves these effectsparticularly effectively in cooperation with Mo. In order to achievethese effects, it is recommended to include 0.005% or more of Nb. Morepreferably 0.01% or more of Nb is added. However, since the effectsdescribed above reach saturation when an excessive Nb content isincluded, resulting in economical disadvantage, Nb content is limited to0.1% or less and more preferably to 0.08% or less.

<B: 0.0002 to 0.01%>

B is effective in increasing the strength of the steel sheet, and it ispreferable that 0.0002% or more (more preferably 0.0005% or more) B iscontained in order to achieve these effects. However, an excessivecontent of B leads to poor hot processing property. Therefore, it ispreferable to control the concentration of B within 0.01% (morepreferably within 0.005%).

<At Least One Kind Selected from Among a Group Consisting of Ca: 0.0005%to 0.005%, Mg: 0.0005% to 0.01% and REM: 0.0005% to 0.01%)

Ca, Mg and REM (rare earth element) are effective in suppressing anincrease in hydrogen ion concentration, that is, a decrease in pH in theatmosphere of the interface due to corrosion of the steel sheet surface,thereby to improve the corrosion resistance of the steel sheet. It isalso effective in controlling the form of sulfide in the steel andimproving the workability of the steel. In order to achieve the effectsdescribed above, it is recommended to add each of Ca, Mg and REM inconcentration of 0.0005% or higher. However, since excessive contents ofthese elements leads to poor workability, it is preferable to keep theconcentrations of Ca within 0.005%, Mg and REM each within 0.01%.

Third Embodiment

A third high strength thin steel sheet according to the presentinvention is constituted from higher than 0.25 and up to 0.60% of C(contents of components given in terms of percentage in this patentapplication all refer to percentage by weight), 1.0 to 3.0% of Si, 1.0to 3.5% of Mn, 0.15% or less of P, 0.02% or less of S, 1.5% or less(higher than 0%) of Al, while iron and inevitable impurities making upthe rest,

wherein: (iii) the structure satisfies the following requirements afterforming:

1% or more residual austenite; the mean axis ratio (major axis/minoraxis) of the residual austenite grains is 5 or higher;

mean length of minor axes of the residual austenite grains is 1 μm orless; and

minimum distance between residual austenite grains is 1 μm or less.

When the metal structure is controlled as described above, hydrogenembrittlement resisting property of the high strength thin steel sheetcan be sufficiently improved without adding much alloy elements.

The phrase “after the forming process” means the state of the steelsheet after being stretch formed with elongation of 3%. Specifically,the steel sheet is subjected to elongation of 3% by uniaxial stretchingat the room temperature (the stretch forming process of 3% elongationmay hereinafter be referred to simply as “process”).

The requirements for the residual austenite of the present inventionwill now be described in detail below.

<1% or More Residual Austenite>

<Mean Axis Ratio (Major Axis/Minor Axis) of the Residual AusteniteGrains is 5 or Higher>

It is necessary that the metal structure contains 1% or more residualaustenite in terms of area proportion to the entire structure afterprocessing, in order to achieve high hydrogen embrittlement resistingproperty in harsh operating environment over an extended period of timeafter forming the part. Residual austenite contributes not only to theimprovement of hydrogen embrittlement resisting property as describedabove, but also to the improvement of total elongation as has been knownin the prior art. Content of the residual austenite is preferably 2% orhigher, and more preferably 3% or higher. Since the desired level ofhigh strength cannot be obtained when an excessive amount of residualaustenite is contained, it is recommended to set an upper limit of 15%(more preferably 10%) to the residual austenite content.

Lath-shaped grains of residual austenite after processing have farhigher capacity of trapping hydrogen than carbide. FIG. 1 is a graphshowing the relationship between the mean axis ratio of the residualaustenite grains measured by a method to be described later and hydrogenembrittlement risk index (measured by a method to be described later inan example, lower value of this index means better hydrogenembrittlement resisting property). From FIG. 1, it can be seen thathydrogen embrittlement risk index sharply decreases when the mean axisratio (major axis/minor axis) of the residual austenite grains increasesbeyond 5. This is supposedly because, when the mean axis ratio of theresidual austenite grains becomes 5 or higher, intrinsic capability ofthe residual austenite to absorb hydrogen is put into full play, so thatthe residual austenite attains far higher capacity of trapping hydrogenthan carbide and substantially neutralizes the hydrogen that infiltratesfrom the outside through atmospheric corrosion thereby to achieveremarkable achievement in hydrogen embrittlement resisting property. Themean axis ratio of the residual austenite grains is preferably 10 orhigher, and more preferably 15 or higher.

<Mean Length of Minor Axes of the Residual Austenite Grains is 1 μm orLess>

According to the present invention, it has been found that hydrogenembrittlement resisting property can be effectively improved bydispersing fine grains of residual austenite of lath shape.Specifically, hydrogen embrittlement resisting property can be surelyimproved by dispersing the lath-shape grains of residual austenitehaving sizes of 1 μm or less (submicrometer order). This is supposedlybecause surface area of the residual austenite grains (interface)increases resulting in larger hydrogen trapping capability, when largernumber of fine lath-shape grains of residual austenite having smallermean length of minor axis are dispersed. Mean length of minor axes ofthe residual austenite grains is preferably 0.5 μm or less, morepreferably 0.25 μm or less.

According to the present invention, hydrogen trapping capability of thefine lath-shape grains of residual austenite can be made far greaterthan that in the case of dispersing carbide, and thereby tosubstantially neutralize hydrogen that infiltrates from the outsidethrough atmospheric corrosion, even when the same proportion by volumeof residual austenite is contained, by controlling the mean axis ratioand mean length of minor axes of the residual austenite grains asdescribed above.

<Minimum Distance Between Residual Austenite Grains is 1 μm or Less>

According to the present invention, it has been found that hydrogenembrittlement resisting property can be improved further by controllingthe minimum distance between adjacent residual austenite grains, inaddition to the above. Specifically, hydrogen embrittlement resistancecan be surely improved when the minimum distance between residualaustenite grains is 1 μm or less. This is supposedly because propagationof cracks is suppressed so that the structure demonstrates higherresistance against fracture, when a large number of fine lath-shapegrains of residual austenite are dispersed in proximity to each other.Minimum distance between adjacent residual austenite grains ispreferably 0.8 μm or less, and more preferably 0.5 μm or less.

The residual austenite refers to a region that is observed as FCC (facecentered cubic lattice) by the FE-SEM/EBSP method which will bedescribed later. Measurement by the EBSP may be done, for example, bymeasuring a measurement area (about 50 by 50 μm) at an arbitrarilychosen position in a surface parallel to the rolled surface at aposition of one quarter of the thickness at measuring intervals of 0.1μm. The measuring surface is prepared by electrolytic polishing in orderto prevent the residual austenite from transforming. Then the test pieceis set in the lens barrel of an FE-SEM equipped with the EBSP detector(of which details will be described later) and is irradiated withelectron beam. An EBSP image projected onto a screen is captured by ahigh sensitivity camera (VE-1000-SIT manufactured by Dage-MTI Inc.) andis sent to a computer. The computer carries out image analysis andgenerates color mapping of the FCC phase through comparison with astructural pattern simulated with a known crystal system (FCC (facecentered cubic lattice) phase in the case of residual austenite). Areaproportion of the region that is mapped as described above is taken asthe area proportion of the residual austenite. This analysis was carriedout by means of hardware and software of OIM (Orientation ImagingMicroscopy™) system of TexSEM Laboratories Inc.

The mean axis ratio, mean length of minor axes and minimum distancebetween residual austenite grains were determined as follows. The meanaxis ratio of the residual austenite grains was determined by measuringthe major axis and minor axis of residual austenite crystal grainexisting in each of three arbitrarily chosen fields of view in theobservation by means of TEM (transmission electron microscope) withmagnification factor of 15000, and averaging the ratios of major axis tominor axis. The mean length of minor axes of the residual austenitegrains was determined by averaging the lengths of minor axes measured asdescribed above. The minimum distance between adjacent residualaustenite grains was determined by measuring the distance betweenadjacent residual austenite grains that were aligned in the direction ofmajor axis as shown as (a) in FIG. 11 (distance (b) in FIG. 11 is notregarded as the minimum distance between the grains) by observing withTEM (magnification factor of 15000) in each of three arbitrarily chosenfields of view and averaging the distances measured in the three fieldsof view.

In order to decrease the number of intergranular fracture initiatingpoints in the steel thereby to surely decrease the concentration ofdiffusive hydrogen to a harmless level and achieve a high strength, itis desirable to form the matrix phase of the steel sheet afterprocessing from a binary phase structure of bainitic ferrite andmartensite with the bainitic ferrite acting as the main phase, insteadof the single phase structure of martensite that is generally used forhigh strength steels.

In the case of single phase structure of martensite, a carbide (forexample, film-like cementite) is likely to precipitate in the grainboundaries, thus making intergranular fracture likely to occur. In thecase of the binary phase structure of bainitic ferrite and martensitewith the bainitic ferrite acting as the main phase, in contrast, thebainitic ferrite is a hard phase and therefore it is easy to increasethe strength of the entire structure as in the case of the single phaseof martensite. The hydrogen embrittlement resisting property can also beimproved as much hydrogen is trapped in the dislocations. It also hassuch an advantage that coexistence of the bainitic ferrite and residualaustenite which will be described later prevents the generation ofcarbide that acts as the intergranular fracture initiating points, andit becomes easier to create the lath-shaped residual austenite in theboundaries of lath-shaped bainitic ferrite.

Accordingly, it is required in the present invention that the binaryphase structure of bainitic ferrite and martensite occupy 80% or more,preferably 85% or more and more preferably 90% or more of the entirestructure after the stretch forming processing to elongate by 3%. Upperlimit of the proportion may be determined by the balance with otherstructure (residual austenite), and is set to 99% when other structure(ferrite, etc.) than the residual austenite is not contained.

The bainitic ferrite referred to in the present invention isplate-shaped ferrite having a lower structure of high density ofdislocations. It is clearly distinguished from polygonal ferrite thathas lower structure including no or very low density of dislocations, bySEM observation as follows.

Area proportion of bainitic ferrite structure is determined as follows.A test piece is etched with Nital etchant. A measurement area (about 50by 50 μm) at an arbitrarily chosen position in a surface parallel to therolled surface at a position of one quarter of the thickness is observedwith SEM (scanning electron microscope) (magnification factor of 1500)thereby to determine the area proportion.

Bainitic ferrite is shown with dark gray color in SEM photograph(bainitic ferrite, residual austenite and martensite may not bedistinguishable in the case of SEM observation), while polygonal ferriteis shown black in SEM photograph and has polygonal shape that does notcontain residual austenite and martensite inside thereof.

The SEM used in the present invention is a high-resolution FE-SEM (FieldEmission type Scanning Electron Microscope XL30S-FEG manufactured byPhilips Inc.) equipped with an EBSP (Electron Back Scatter diffractionPattern) detector, that has a merit of being capable of analyzing thearea observed by the SEM at the same time by means of the EBSP detector.EBSP detection is carried out as follows. When the sample surface isirradiated with electron beam, the EBSP detector analyzes the Kikuchipattern obtained from the reflected electrons, thereby to determine thecrystal orientation at the point where the electron beam has hit upon.Distribution of orientations over the sample surface can be measured byscanning the electron beam two-dimensionally over the sample surfacewhile measuring the crystal orientation at predetermined intervals. TheEBSP detection method has such an advantage that different structuresthat are regarded as the same structure in the ordinary microscopicobservation but have different crystal orientations can be distinguishedby the difference in color tone.

The metal structure after the processing may be constituted either fromonly the structures described above (namely, a mixed structure ofbainitic ferrite+martensite and residual austenite), or may includeother structure such as ferrite (the term ferrite used herein refers topolygonal ferrite, that is a ferrite structure that includes no or veryfew dislocations) or pearlite to such an extent that the effect of thepresent invention is not compromised. Such additional components arestructures that can inevitably remain in the manufacturing process ofthe present invention, of which concentration is preferably as low aspossible, within 9%, preferably less than 5% and more preferably lessthan 3% according to the present invention.

In order to maintain high hydrogen embrittlement resisting propertyafter the forming process, for example, high proportion of residualaustenite, 5% or more, may be contained in the steel sheet prior to theforming process, or a large amount of fine residual austenite grains maybe dispersed in the structure. Alternatively, forming process conditionsmay be controlled so as to make the residual austenite less likely totransform (for example, form the part by bending operation or controlthe forming temperature and/or stretching speed). The most desirablemeans of improving the workability and hydrogen embrittlement resistingproperty at the same time while maintaining the content of residualaustenite before and after the processing substantially constant withinan appropriate range and maintaining other properties (high strength,etc.) is to satisfy the requirements (A) and (B) described previously.

While this embodiment is characterized in that the metal structure iscontrolled after processing, it is necessary to control the othercomponents as described previously, in order to form the metal structureand efficiently improve hydrogen embrittlement resisting property andstrength thereby to ensure the level of ductility required for the thinsteel sheet.

While the present invention does not specify the manufacturingconditions, it is recommended to apply heat treatment in the followingprocedure after hot rolling or cold rolling conducted thereafter, inorder to form the structure described above that can be easily workedand has high strength and high hydrogen embrittlement resistance afterthe processing, by using the steel material of the composition describedabove. The recommended procedure is to keep the steel the compositiondescribed above at a temperature (T1) in a range from A3 point to (A3point+50° C.) for a period of 10 to 1800 seconds (t1), cool down thesteel at a mean cooling rate of 3° C./s or higher to a temperature (T2)in a range from Ms point to Bs point and keep the material at thistemperature for a period of 60 to 3600 seconds (t2).

It is not desirable that the temperature T1 becomes higher than (A3point+50° C.) or the period t1 is longer than 1800 seconds, in whichcase austenite grains grow resulting in poor workability (elongationflanging property). When the temperature T1 is lower than A3 point, onthe other hand, desirable bainitic ferrite structure cannot be obtained.When the period t1 is shorter than 10 seconds, austenitization does notproceed sufficiently and therefore cementite and other alloy carbidesremain. The period t1 is preferably in a range from 30 to 600 seconds,more preferably from 60 to 400 seconds.

Then the steel sheet is cooled down. The steel is cooled at a meancooling rate of 3° C./s or higher, for the purpose of preventingpearlite structure from being generated while avoiding the pearlitetransformation region. The mean cooling rate should be as high aspossible, and is preferably 5° C./s or higher, and more preferably 10°C./s or higher.

After quenching to the temperature between Ms point and Bs point at therate described above, the steel is subjected to isothermaltransformation so as to transform the matrix phase into binary phasestructure of bainitic ferrite and martensite. When the heat retainingtemperature T2 is higher than Bs, much pearlite that is not desirablefor the present invention is formed, thus hampering the formation of thepredetermined bainitic ferrite structure. When T2 is below Ms, on theother hand, the amount of residual austenite decreases.

When the temperature holding period t2 is longer than 1800 seconds,density of dislocations in bainitic ferrite becomes low, the amount oftrapped hydrogen decreases and the desired residual austenite cannot beobtained. When t2 is less than 60 second, on the other hand, desiredbainitic ferrite structure cannot be obtained. The length of t2 ispreferably from 90 to 1200 seconds, and more preferably from 120 to 600seconds. There is no restriction on the method of cooling aftermaintaining the heating temperature, and air cooling, quenching orair-assisted water cooling may be employed.

In the practical manufacturing process, the annealing process describedabove can be carried out easily by employing a continuous annealingfacility or a batch annealing facility. In case that a cold rolled sheetis plated with zinc by hot dipping, the heat treatment process may bereplaced by the plating process by setting the plating conditions so asto satisfy the heat treatment conditions. The plating may also bealloyed.

There is no restriction on the hot rolling process (or cold rollingprocess as required) that precedes the continuous annealing processdescribed above, and commonly employed process conditions may be used.Specifically, the hot rolling process may be carried out in such aprocedure as, after hot rolling at a temperature above Ar3 point, thesteel sheet is cooled at a mean cooling rate of about 30° C./s and iswound up at a temperature from about 500 to 600° C. In case that the hotrolled steel sheet has unsatisfactory appearance, cold rolling may beapplied in order to rectify the appearance. It is recommended to set thecold rolling ratio in a range from 1 to 70%. Cold rolling beyond 70%leads to excessive rolling load that makes it difficult to carry out thecold rolling.

While the present invention is addressed to thin steel sheet, there isno limitation to the form of product, and may be applied, in addition tosteel sheet made by hot rolling or steel sheet made by cold rolling, tothose subjected to annealing after hot rolling or cold rolling, followedby chemical conversion treatment, hot-dip coating, electroplating, vapordeposition, painting, priming for painting, organic coating treatment orthe like.

The plating process may be either galvanizing or aluminum plating. Themethod of plating may be either hot-dip coating or electroplating, andthe plating process may also be followed by alloying heat treatment ormulti-layer plating. A steel sheet, that is plated or not plated, mayalso be laminated with a film.

When the coating operation described above is carried out, chemicalconversion treatment such as phosphating or electrodepositing coatingmay be applied in accordance to the application. The coating materialmay be a known resin that can be used in combination with a knownhardening agent such as epoxy resin, fluorocarbon resin, siliconeacrylic resin, polyurethane resin, acrylic resin, polyester resin,phenol resin, alkyd resin, or melamine resin. Among these, epoxy resin,fluorocarbon resin or silicone acrylic resin is preferably used inconsideration of corrosion resistance. Known additives that are added tocoating materials such as coloring agent, coupling agent, levelingagent, sensitization agent, antioxidant agent, anti-UV protection agent,flame retarding agent or the like may be used.

There is also no restriction on the coating and solvent-based coating,powder coating, water-based coating, water-dispersed coating,electrodeposition coating or like may be employed. Desired coating layerof the coating material described above can be formed on the steel by aknown technique such as dipping, roll coater, spraying, or curtain flowcoater. The coating layer may have any proper thickness.

The high strength thin steel sheet of the present invention may beapplied to high-strength automotive components such as bumper, doorimpact beam, pillar and other reinforcement members and interior partssuch as seat rail, etc. Automobile components that are manufactured byforming process also have sufficient properties (strength) and highhydrogen embrittlement resisting property.

The present invention will now be described below by way of examples,but the present invention is not limited to the examples. Variousmodifications may be conceived without departing from the technicalscope of the present invention.

Example 1

Sample steels A-1 through Y-1 having the compositions described in Table1 were melt-refined in vacuum to make test slabs. The slabs wereprocessed in the following procedure (hot rolling→coldrolling→continuous annealing) thereby to obtain hot-rolled steel platesmeasuring 3.2 mm in thickness. The steel plates were pickled to removescales from the surface and then cold rolled so as to reduce thethickness to 1.2 mm.

<Hot rolling> Starting temperature (SRT): Held at a temperature between1150 and 1250° C. for 30 minutes.

Finishing temperature (FDT): 850° C.

Cooling rate: 40° C./s

Winding-up temperature: 550° C.

<Cold rolling> Rolling ratio: 50%

<Continuous annealing> Each steel specimen was kept at a temperature ofA3 point+30° C. for 120 seconds, then cooled in air at a mean coolingrate of 20° C./s to temperature T0 shown in Table 2, and was kept at T0for 240 seconds, followed by air-assisted water cooling to the roomtemperature.

No. 116 shown in Table 2 was made by heating a cold-rolled steel sheetto 830° C., keeping at this temperature for 5 minutes followed byquenching in water and tempering at 300° C. for 10 minutes, thereby toform a martensite steel as a comparative example of the high-strengthsteel of the prior art. No. 120 was made by heating a cold-rolled steelsheet to 800° C., keeping at this temperature for 120 seconds, coolingdown at a mean cooling rate of 20° C./s to 350° C. and keeping at thistemperature for 240 seconds.

JIS No. 5 test pieces were prepared from the steel sheets obtained asdescribed above, and were subjected to stretch forming process withelongation of 3% mimicking the actual manufacturing process. Metalstructures of the test pieces were observed before and after theprocessing, tensile strength (TS) and elongation (total elongation E1)before the processing and hydrogen embrittlement resisting propertyafter the processing were measured by the following procedures.

Observation of Metal Structure

Metal structures of the test pieces were observed before and after theprocessing as follows. A measurement area (about 50 by 50 μm) at anarbitrarily chosen position in a surface parallel to the rolled surfaceat a position of one quarter of the thickness was photographed atmeasuring intervals of 0.1 μm, and area proportions of bainitic ferrite(BF), martensite (M) and residual austenite (residual γ) were measuredby the method described previously. Then similar measurements were madein two fields of view that were arbitrarily selected, and the measuredvalues were averaged. Area proportions of other structures (ferrite,pearlite, etc.) were subtracted from the entire structure.

Mean axis ratio of the residual austenite grains of the steel sheetbefore and after the processing were measured by the method describedpreviously. Test pieces having mean axis ratio of 5 or higher wereregarded to satisfy the requirements of the present invention (∘), andthose having mean axis ratio of lower than 5 were regarded to fail tosatisfy the requirements of the present invention (x).

Measurement of tensile strength (TS) and elongation (E1) Tensile testwas conducted on the JIS No. 5 test piece before processing, so as tomeasure the tensile strength (TS) and elongation (E1). Stretching speedof the tensile test was set to 1 mm/sec. Among the steel sheets havingtensile strength of 1180 MPa as measured by the method describedpreviously, those which showed elongation of 10% or more were evaluatedas high in elongation property.

Evaluation of Hydrogen Embrittlement Resisting Property

In order to evaluate hydrogen embrittlement resisting property, the JISNo. 5 test piece was stretched so as to elongate by 3%. Then afterbending with a radius of curvature of 15 mm, load of 1000 MPa wasapplied and the test piece was immersed in 5% solution of hydrochloricacid, and the time before crack occurred was measured.

Hydrogen-charged 4-point bending test was also conducted for some steelspecies. Specifically, a rectangular test piece measuring 65 mm by 10 mmmade of each steel sheet elongated by 3% was immersed in a solution of0.5 mol of H₂SO₄ and 0.01 mol of KSCN and was subjected to cathodehydrogen charging. Maximum stress endured without breaking for 3 hourswas determined as the critical fracture stress (DFL).

Results of these tests are shown in Table 2. TABLE 1 Steel speciesChemical composition (mass %)* Ac3 Bs Ms Symbol C Si Mn P S Al Mo NbOthers (° C.) (° C.) (° C.) A-1 0.40 2.01 2.01 0.012 0.002 0.033 0.20.05 — 839.1 524.5 300.9 B-1 0.27 2.02 2.54 0.011 0.002 0.031 0.2 0.06 —845.0 511.9 345.0 C-1 0.55 2.00 2.54 0.011 0.002 0.031 0.2 0.06 — 799.1436.3 212.3 D-1 0.39 2.55 2.51 0.011 0.002 0.030 0.2 0.05 — 847.9 482.2289.1 E-1 0.41 2.01 1.32 0.011 0.002 0.031 0.2 0.04 — 856.7 583.9 318.9F-1 0.40 2.02 3.15 0.011 0.002 0.030 0.2 0.05 — 803.4 421.9 263.3 G-10.40 2.02 2.51 0.012 0.002 0.031 0.8 0.05 — 842.6 429.7 271.8 H-1 0.401.98 2.48 0.011 0.002 0.033 0.2 0.08 — 822.9 482.2 285.4 I-1 0.40 2.022.50 0.011 0.002 0.031 0.2 0.05 B: 0.0005 823.3 480.4 284.7 J-1 0.401.98 2.53 0.011 0.002 0.033 0.2 0.05 Ca: 0.004 821.4 477.7 283.7 K-10.40 1.49 2.48 0.012 0.002 0.033 0.2 0.05 Mg: 0.005 801.7 482.2 285.4L-1 0.39 1.51 2.50 0.011 0.002 0.032 0.2 0.06 REM: 0.005 802.5 483.1289.4 M-1 0.35 1.50 2.50 0.011 0.002 0.033 0.2 0.05 Ca: 0.005, Mg:0.005, 809.2 493.9 308.4 B: 0.0005 N-1 0.19 1.51 1.52 0.014 0.005 0.0310.2 0.05 — 871.9 625.3 416.6 O-1 0.30 1.40 0.90 0.011 0.005 0.032 0.20.05 — 861.2 651.4 384.9 P-1 0.35 0.16 1.42 0.014 0.002 0.043 0.2 0.05 —787.8 591.1 344.0 Q-1 0.7 2.01 2.01 0.013 0.003 0.033 0.2 0.05 — 798.3443.5 158.7 R-1 0.40 2.02 1.60 0.012 0.002 0.033 1.5 0.05 — 892.8 453.5287.1 S-1 0.40 2.00 1.20 0.012 0.002 0.031 0.2 0.2 — 862.1 597.4 327.6T-1 0.27 2.01 2.50 0.011 0.002 0.053 0.2 0.05 — 854.6 515.5 346.3 U-10.28 2.02 2.50 0.012 0.002 0.263 0.2 0.05 — 937.8 512.8 341.6 V-1 0.282.02 2.51 0.011 0.002 0.418 0.2 0.05 Ca: 0.004 998.8 511.9 341.3 W-10.28 2.02 2.51 0.011 0.002 0.681 0.2 0.05 Ca: 0.004 1104.0 511.9 341.3X-1 0.27 1.98 2.55 0.012 0.002 1.01 0.2 0.05 Mg: 0.005, B: 0.0005 1235.2511.0 344.7 Y-1 0.28 2.02 2.60 0.011 0.002 1.68 0.2 0.05 — 1500.9 503.8338.3*The balance consists of iron and inevitable impurities.

TABLE 2 Before processing After processing Steel Mean axis Mean axisHydrogen species To Residual γ BF + M Others ratio of TS El Residual γBF + M Others ratio of embrittlement DFL Test No. Symbol ° C. % % %residual γ MPa % % % % residual γ h MPa 101 A-1 350 9 91 0 ∘ 1510 14 595 0 ∘ Over 24 — 102 B-1 350 7 93 0 ∘ 1495 12 3 97 0 ∘ Over 24 420 103C-1 350 14 86 0 ∘ 1521 13 5 95 0 ∘ Over 24 — 104 D-1 350 12 88 0 ∘ 149513 4 96 0 ∘ Over 24 — 105 E-1 350 11 89 0 ∘ 1417 14 4 96 0 ∘ Over 24 —106 F-1 350 12 88 0 ∘ 1376 14 5 95 0 ∘ Over 24 — 107 G-1 350 11 88 1 ∘1433 14 4 96 0 ∘ Over 24 — 108 H-1 350 11 88 1 ∘ 1476 14 3 97 0 ∘ Over24 — 109 I-1 350 12 88 0 ∘ 1486 13 4 96 0 ∘ Over 24 — 110 J-1 350 11 890 ∘ 1491 12 3 97 0 ∘ Over 24 — 111 K-1 350 12 88 0 ∘ 1488 13 4 96 0 ∘Over 24 — 112 L-1 350 12 88 0 ∘ 1487 14 3 97 0 ∘ Over 24 — 113 M-1 35011 89 0 ∘ 1454 13 4 96 0 ∘ Over 24 — 114 N-1 430 7 93 0 ∘ 1417 8 <1 99<1 ∘ Over 24 — 115 O-1 350 <1 99 <1 x 1436 8 <1 99 <1 x 11 — 116 P-1 —<1 99 <1 x 1503 4 <1 99 <1 x 7 — 117 Q-1 350 15 85 0 ∘ 1511 3 3 97 0 ∘ 3— 118 R-1 350 11 89 0 ∘ 1341 1 — — — — — — 119 S-1 350 11 89 0 ∘ 1370 2— — — — — — 120 A-1 350 8 20 72 x 960 13 15 85 0 x Over 24 — 121 T-1 3507 93 0 ∘ 1495 12 3 97 0 ∘ Over 24 515 122 U-1 350 8 92 0 ∘ 1499 12 4 960 ∘ Over 24 650 123 V-1 350 8 92 0 ∘ 1503 13 4 96 0 ∘ Over 24 680 124W-1 350 8 92 0 ∘ 1505 13 5 95 0 ∘ Over 24 700 125 X-1 350 8 92 0 ∘ 149813 5 95 0 ∘ Over 24 715 126 Y-1 350 12 59 29 x 1290 14 9 91 0 x 8 290

The results shown in Tables 1 and 2 can be interpreted as follows(numbers in the following description are test Nos. in Table 2).

Test pieces Nos. 101 through 113 (inventive steel sheets 2) and testpieces Nos. 121 through 125 (inventive steel sheets 1) that satisfy therequirements of the present invention have high strength of 1180 MPa orhigher, and high hydrogen embrittlement resisting property in harshenvironment after the forming process. They also have high elongationproperty required of the TRIP steel sheet, thus providing steel sheetsbest suited for reinforcement parts of automobiles that are exposed tocorrosive atmosphere. Test pieces Nos. 121 through 125, in particular,show even better hydrogen embrittlement resisting property.

Test pieces Nos. 114 through 120 and 126 that do not satisfy therequirements of the present invention, in contrast, have the followingdrawbacks.

No. 114 made of steel species N-1 that includes insufficient C contentdoes not have good workability.

No. 115 made of steel species O-1 that includes insufficient Mn contentdoes not retain sufficient residual austenite and is inferior inhydrogen embrittlement resisting property after the processing.

No. 116, martensite steel that is a conventional high strength steelmade of steel species P-1 that includes insufficient Si content, hardlycontains residual austenite and is inferior in hydrogen embrittlementresisting property. It also does not show the elongation propertyrequired of a thin steel sheet.

No. 117 made of steel species Q-1 that includes excessive C content hasprecipitation of carbide and is inferior in both forming workability andhydrogen embrittlement resisting property after processing.

No. 118 made of steel species R-1 that includes excessive Mo content andNo. 119 made of steel species S-1 that includes excessive Nb content areinferior in forming workability. Nos. 118 and 119 could not undergo theprocessing, making it impossible to investigate the property after theprocessing.

No. 120, that was made of a steel that has the composition specified inthe present invention but was not manufactured under the recommendedconditions, resulted in the conventional TRIP steel. As a result, theresidual austenite does not have the mean axis ratio specified in thepresent invention, while the matrix phase is not formed in binary phasestructure of bainitic ferrite and martensite, and therefore sufficientlevel of hydrogen embrittlement resisting property is not achieved.

No. 126 includes Al content higher than that specified for the inventivesteel sheet 1. As a result, although the predetermined amount ofresidual austenite is retained, the residual austenite does not have themean axis ratio specified in the present invention, the desired matrixphase is not obtained and inclusions such as AlN are generated thusresulting in poor hydrogen embrittlement resisting property.

Then parts were made by using steel species A-1, J-1 shown in Table 1and comparative steel sheet (590 MPa class high strength steel sheet ofthe prior art). Performance (pressure collapse resistance and impactresistance) of the formed test piece were studied by conducting pressurecollapse test and impact resistance test as follows.

Pressure Collapse Test

The part 1 (hat channel as test piece) shown in FIG. 1 was made by usingsteel species A-1, J-1 shown in Table 1 and the comparative steel sheet,and was subjected to pressure collapse test. The part was spot welded atthe positions 2 of the part shown in FIG. 1 at 35 mm intervals as shownin FIG. 1 by supplying electric current of a magnitude less than theexpulsion generating current by 0.5 kA from an electrode measuring 6 mmin diameter at the distal end. Then a die 3 was pressed against the part1 from above the mid portion thereof in the longitudinal direction asshown in FIG. 2, and the maximum tolerable load was determined. Absorbedenergy was determined from the area under the load-deformation curve.The results are shown in Table 3. TABLE 3 Evaluation of test piece Steelsheet used Maximum Energy TS EL Residual γ load absorbed Steel species(MPa) (%) (Area %) (kN) (kJ) Symbol A-1 1510 15 9 14.1 0.72 Symbol J-11491 12 11 14 0.68 Comparative steel 613 22 0 5.7 0.33 sheet

From Table 3, it can be seen that the part (test piece) made from thesteel sheet of the present invention has higher load bearing capabilityand absorbs greater energy than a part made of the conventional steelsheet having lower strength, thus showing high pressure collapseresistance.

Impact Resistance Test

The parts 4 (hat channel as test piece) shown in FIG. 3 were made byusing steel species A-1, J-1 shown in Table 1 and the comparative steelsheet, and were subjected to impact resistance test. FIG. 4 is asectional view along A-A of the part 4 shown in FIG. 3. In the impactresistance test, after the part was spot welded at the positions 5 ofthe part 4 similarly to the pressure collapse test, the part 4 wasplaced on a base 7 as schematically shown in FIG. 5. A weight 6(weighing 10 kg) was dropped onto the part 4 from a height of 11 meters,and the energy absorbed before the part 4 underwent deformation of 40 mmin the direction of height. The results are shown in Table 4. TABLE 4Evaluation of Steel sheet used test piece TS EL Residual γ Energyabsorbed Steel species (MPa) (%) (Area %) (kJ) Symbol A-1 1510 15 9 6.94Symbol J-1 1491 12 11 6.65 Comparative 613 22 0 3.56 steel sheet

From Table 4, it can be seen that the part (test piece) made from thesteel sheet of the present invention absorbs greater energy than a partmade of the conventional steel sheet that has lower strength, thusshowing higher impact resistance.

TEM photograph of the test piece made in this example is shown asreference. FIG. 6 is a photograph of TEM observation of No. 101 of thepresent invention. From FIG. 6, it can be seen that the high strengththin steel sheet of the present invention contains lath-shaped residualaustenite (black portion of bar shape in FIG. 6) specified in thepresent invention dispersed therein. FIG. 7 is a photograph of TEMobservation of No. 120 of a comparative example. From FIG. 7, it can beseen that the high strength thin steel sheet of No. 120 containsresidual austenite (black portion of somewhat round shape in FIG. 7),although the residual austenite has a block shape that does not satisfythe requirements of the present invention.

Example 2

Sample steels A-2 through Y-2 having the compositions described in Table5 were melt-refined in vacuum to make test slabs. The slabs wereprocessed in the following procedure (hot rolling→coldrolling→continuous annealing) thereby to obtain hot-rolled steel platesmeasuring 3.2 mm in thickness. The steel plates were pickled to removescales from the surface and then cold rolled so as to reduce thethickness to 1.2 mm.

<Hot Rolling>

Starting temperature (SRT): Held at a temperature between 1150 and 1250°C. for 30 minutes.

Finishing temperature (FDT): 850° C.

Cooling rate: 40° C./s

Winding-up temperature: 550° C.

<Cold Rolling>

Rolling ratio: 50%

<Continuous Annealing>

Each steel specimen was kept at a temperature of A3 point+30° C. for 120seconds, then rapidly cooled (air cooling) at a mean cooling rate of 20°C./s to temperature T0 shown in Table 6, and was kept at T0 for 240seconds, followed by air-assisted water cooling to the room temperature.

No. 217 in Table 6 was made by heating a cold-rolled steel sheet to 830°C., keeping at this temperature for 5 minutes followed by quenching inwater and tempering at 300° C. for 10 minutes, thereby to form amartensite steel as a comparative example of the high-strength steel ofthe prior art. No. 220 was made by heating a cold-rolled steel sheet to800° C., keeping at this temperature for 120 seconds, cooling down at amean cooling rate of 20° C./s to 350° C. and keeping at this temperaturefor 240 seconds.

JIS No. 5 test pieces were prepared from the steel sheets obtained asdescribed above, and were subjected to stretch forming process withelongation of 3% mimicking the actual manufacturing process. Metalstructures of the test pieces were observed before and after theprocessing, tensile strength (TS) and elongation (total elongation E1)before the processing and hydrogen embrittlement resisting propertyafter the processing were measured by the following procedures.

Observation of Metal Structure

Metal structures of the test pieces were observed before and after theprocessing as follows. A measurement area (about 50 by 50 μm) at anarbitrarily chosen position in a surface parallel to the rolled surfaceat a position of one quarter of the thickness was photographed atmeasuring intervals of 0.1 μm, and area proportions of bainitic ferrite(BF), martensite (M) and residual austenite (residual γ) were measuredby the method described previously. Then similar measurements were madein two fields of view that were arbitrarily selected, and the measuredvalues were averaged. Area proportions of other structures (ferrite,pearlite, etc.) were subtracted from the entire structure.

Mean axis ratio of the residual austenite grains of the steel sheetbefore and after the processing were measured by the method describedpreviously. Test pieces having mean axis ratio of 5 or higher wereregarded to satisfy the requirements of the present invention (∘), andthose having mean axis ratio of lower than 5 were regarded to fail tosatisfy the requirements of the present invention (x).

Measurement of Tensile Strength (TS) and Elongation (E1)

Tensile test was conducted on the JIS No. 5 test piece beforeprocessing, so as to measure the tensile strength (TS) and elongation(E1). Stretching speed of the tensile test was set to 1 mm/sec. Amongthe steel sheets having tensile strength of 1180 MPa as measured by themethod described previously, those which showed elongation of 10% ormore were evaluated as high in elongation property.

Evaluation of Hydrogen Embrittlement Resisting Property

In order to evaluate the hydrogen embrittlement resisting property, theJIS No. 5 test piece was stretched so as to elongate by 3%. Then afterbending with a radius of curvature of 15 mm, load of 1000 MPa wasapplied and the test piece was immersed in 5% solution of hydrochloricacid, to measure the time before crack occurred.

The bent test pieces prepared as described above were subjected toaccelerated exposure test in which 3% solution of NaCl was sprayed onceevery day for 30 days simulating the actual operating environment, andthe number of days before crack occurred was determined.

Hydrogen-charged 4-point bending test was also conducted for some steelspecies. Specifically, a rectangular test piece measuring 65 mm by 10 mmmade of each steel sheet elongated by 3% was immersed in a solution of0.5 mol of H₂SO₄ and 0.01 mol of KSCN and was subjected to cathodehydrogen charging. Maximum stress endured without breaking for 3 hourswas determined as the critical fracture stress (DFL). Then the ratio(DFL ratio) of this value to the value of DFL of test No. 203 (steelspecies C-2) shown in Table 6 was determined.

Results of these tests are shown in Table 6. TABLE 5 Steel speciesChemical composition (mass %)* Ac3 Bs Ms Symbol C Si Mn P S Al Cu Ni TiV Nb Ma Others ° C. ° C. ° C. A-2 0.40 2.03 2.01 0.012 0.002 0.033 0.3 —— — — — — 827.7 541.1 305.1 B-2 0.27 2.02 2.48 0.011 0.002 0.031 — 0.3 —— — — — 836.0 522.8 346.1 C-2 0.40 2.55 2.51 0.011 0.002 0.030 — 0.3 — —— — — 835.4 485.0 283.5 D-2 0.39 2.01 1.25 0.011 0.002 0.031 — 0.3 — — —— — 851.1 601.1 329.8 E-2 0.41 2.00 3.15 0.011 0.002 0.030 — 0.3 — — — —— 790.1 424.7 257.6 F-2 0.40 1.99 2.53 0.011 0.002 0.033 0.3 0.05 — — —— — 808.8 492.5 287.1 G-2 0.41 2.01 2.50 0.011 0.002 0.031 0.3 0.2 0.05— — — — 825.9 486.9 280.8 H-2 0.40 1.98 2.48 0.011 0.002 0.033 0.3 0.2 —0.05 — — — 812.8 491.4 286.2 I-2 0.40 2.02 2.50 0.011 0.002 0.031 0.30.2 0.05 0.05 — — — 833.2 489.6 285.5 J-2 0.40 1.51 2.48 0.011 0.0020.033 0.3 0.2 0.035 — — — Zr: 0.02 800.6 491.4 286.2 K-2 0.39 2.02 2.500.011 0.002 0.032 0.3 0.2 0.05 — 0.05 0.2 — 836.3 475.7 286.0 L-2 0.402.01 2.50 0.011 0.002 0.033 0.3 0.2 0.05 — 0.06 0.2 — 834.6 473.0 281.3M-2 0.35 1.50 2.50 0.012 0.002 0.033 0.3 0.2 0.05 — 0.05 0.2 B: 0.0005820.8 486.5 305.0 N-2 0.55 1.52 1.53 0.012 0.002 0.033 0.3 0.2 0.05 —0.05 0.2 Ca: 0.004, 820.4 519.8 242.2 Mg: 0.005 O-2 0.19 1.51 1.52 0.0110.005 0.031 — 0.2 — — — — — 860.5 634.5 417.4 P-2 0.30 1.40 0.90 0.0110.005 0.032 — 0.2 — — — — — 851.9 660.6 385.7 Q-2 0.35 0.16 1.42 0.0120.002 0.043 — 0.2 — — — — — 777.0 600.3 344.8 R-2 0.7 2.01 2.01 0.0120.002 0.033 — 0.2 — — — — — 788.3 452.7 159.5 S-2 0.38 2.02 1.20 0.0120.002 0.033 — — — — — — — 860.8 619.4 341.3 T-2 0.40 2.02 2.01 0.0120.002 0.050 0.3 0.2 — — — — — 831.0 533.7 301.7 U-2 0.39 2.01 2.02 0.0110.002 0.250 0.3 0.2 0.05 — — — — 931.1 535.5 306.1 V-2 0.41 1.98 2.000.012 0.002 0.411 0.3 0.2 0.05 — 0.05 0.2 — 998.6 515.3 293.1 W-2 0.402.01 1.99 0.012 0.002 0.761 0.3 0.2 0.05 — 0.05 0.2 — 1141.8 518.9 298.1X-2 0.39 2.00 2.00 0.012 0.002 1.03 0.3 0.2 0.05 — 0.05 0.2 Ca: 0.004,1250.3 520.7 302.5 Mg: 0.005 Y-2 0.41 1.98 2.01 0.011 0.002 1.76 0.3 0.20.05 — 0.05 0.2 — 1537.2 514.4 292.7*The balance consists of iron and inevitable impurities.

TABLE 6 Before processing After processing Mean Mean Hydrochloric axisaxis acid Steel ratio of ratio of immersion Exposure Test species ToResidual γ BF + M Others residual TS El Residual γ BF + M Othersresidual test test DFL No. Symbol ° C. % % % γ MPa % % % % γ h Daysratio 201 A-2 400 13 87 0 ∘ 1512 14 6 94 0 ∘ Over 24 Over 30 — 202 B-2400 11 89 0 ∘ 1233 16 4 96 0 ∘ Over 24 Over 30 — 203 C-2 350 12 88 0 ∘1468 13 6 94 0 ∘ Over 24 Over 30 1.00 204 D-2 350 11 88 1 ∘ 1510 13 4 960 ∘ Over 24 Over 30 — 205 E-2 350 12 88 0 ∘ 1467 14 6 94 0 ∘ Over 24Over 30 — 206 F-2 350 12 88 0 ∘ 1491 13 5 95 0 ∘ Over 24 Over 30 — 207G-2 350 12 88 0 ∘ 1492 13 4 96 0 ∘ Over 24 Over 30 — 208 H-2 350 12 88 0∘ 1505 14 4 96 0 ∘ Over 24 Over 30 — 209 I-2 350 12 88 0 ∘ 1501 13 3 970 ∘ Over 24 Over 30 — 210 J-2 350 13 87 0 ∘ 1461 14 5 95 0 ∘ Over 24Over 30 — 211 K-2 350 13 87 0 ∘ 1485 14 5 95 0 ∘ Over 24 Over 30 — 212L-2 350 13 87 0 ∘ 1495 13 3 97 0 ∘ Over 24 Over 30 — 213 M-2 350 11 88 1∘ 1490 14 4 96 0 ∘ Over 24 Over 30 — 214 N-2 350 14 86 0 ∘ 1478 12 5 950 ∘ Over 24 Over 30 — 215 O-2 430 6 92 2 ∘ 1410 11 <1 99 <1 ∘ 9 7 — 216P-2 400 <1 99 <1 x 1432 8 <1 99 <1 x 8 6 — 217 Q-2 — <1 99 <1 x 1487 3<1 99 <1 x 3 3 — 218 R-2 350 14 86 0 ∘ 1495 6 5 95 0 ∘ 17 8 — 219 S-2350 11 88 1 ∘ 1448 8 3 97 0 ∘ Over 24 14 — 220 A-2 350 9 20 71 x 961 1415 85 0 x Over 24 Over 30 — 221 T-2 400 11 89 0 ∘ 1490 14 7 93 0 ∘ Over24 Over 30 1.19 222 U-2 400 12 88 0 ∘ 1498 13 8 92 0 ∘ Over 24 Over 301.49 223 V-2 400 12 88 0 ∘ 1509 14 8 92 0 ∘ Over 24 Over 30 1.58 224 W-2400 12 88 0 ∘ 1511 14 9 91 0 ∘ Over 24 Over 30 1.58 225 X-2 400 12 88 0∘ 1503 13 8 92 0 ∘ Over 24 Over 30 1.63 226 Y-2 400 15 55 30 x 1290 1511 89 0 x 14 11 0.79

The results shown in Tables 5 and 6 can be interpreted as follows(numbers in the following description are test Nos. in Table 6).

Test pieces Nos. 201 through 214 (inventive steel sheets 2) and testpieces Nos. 221 through 225 (inventive steel sheets 1) that satisfy therequirements of the present invention have high strength of 1180 MPa orhigher, and high hydrogen embrittlement resisting property in harshenvironment after the forming process. They also have high elongationproperty required of the TRIP steel sheet, thus providing steel sheetsbest suited for reinforcement parts of automobiles that are exposed tocorrosive atmosphere. Test pieces Nos. 221 through 225, in particular,show even better hydrogen embrittlement resisting property.

Test pieces Nos. 215 through 220 and 226 that do not satisfy therequirements of the present invention, in contrast, have the followingdrawbacks.

No. 215 made of steel species O-2 that includes insufficient C contenthas the amount of residual austenite significantly decreased after theprocessing, and fails to show the required level of hydrogenembrittlement resisting property of the present invention.

No. 216 made of steel species P-2 that includes insufficient Mn contentdoes not retain sufficient residual austenite and is inferior inhydrogen embrittlement resisting property after the processing.

No. 217, martensite steel that is a conventional high strength steelmade of steel species Q-2 that includes insufficient Si content, hardlycontains residual austenite and is inferior in hydrogen embrittlementresisting property. It also does not show the elongation propertyrequired of a thin steel sheet.

No. 218 made of steel species R-2 that includes excessive C content hasprecipitation of carbide and is inferior in both the forming workabilityand the hydrogen embrittlement resisting property after processing.

No. 219 made of steel species S-2 that does not include Cu and/or Nishows insufficient corrosion resistance and fails to show the requiredlevel of hydrogen embrittlement resisting property of the presentinvention.

No. 220, that was made of a steel that has the composition specified inthe present invention but was not manufactured under the recommendedconditions, resulted in the conventional TRIP steel. As a result, theresidual austenite does not have the mean axis ratio specified in thepresent invention, while the matrix phase is not formed in binary phasestructure of bainitic ferrite and martensite, and therefore sufficientlevel of hydrogen embrittlement resisting property is not achieved.

No. 226 includes Al content higher than that specified for the inventivesteel sheet 1. As a result, although the predetermined amount ofresidual austenite is retained, the residual austenite does not have themean axis ratio specified in the present invention, the desired matrixphase is not obtained and inclusions such as AlN are generated thusresulting in poor hydrogen embrittlement resisting property.

Then parts were made by using steel species A-2, K-2 shown in Table 5and comparative steel sheet (590 MPa class high strength steel sheet ofthe prior art). Performance (pressure collapse resistance and impactresistance) of the formed test piece were studied by conducting pressurecollapse test and impact resistance test as follows.

Pressure Collapse Test

Maximum tolerable load was determined similarly to Example 1 by usingsteel species A-2, K-2 shown in Table 5 and the comparative steel sheet.Absorbed energy was determined from the area lying under theload-deformation curve. The results are shown in Table 7. TABLE 7Evaluation of test piece Steel sheet used Maximum Energy TS EL Residualγ load absorbed Steel species (MPa) (%) (Area %) (kN) (kJ) Symbol A-21512 14 13 14.1 0.7 Symbol K-2 1485 14 13 13.9 0.68 Comparative steel613 22 0 5.7 0.33 sheet

From Table 7, it can be seen that the part (test piece) made from thesteel sheet of the present invention has higher load bearing capabilityand absorbs greater energy than a part made of the conventional steelsheet that has lower strength, thus showing higher pressure collapseresistance.

Impact Resistance Test

The impact resistance test was conducted similarly to Example 1 on thesteel sheets made of steel species A-2, K-2 shown in Table 5 and thecomparative steel sheet. The results are shown in Table 8. TABLE 8Evaluation of test Steel sheet used piece TS EL Residual γ Energyabsorbed Steel species (MPa) (%) (Area %) (kJ) Symbol A-2 1512 14 137.06 Symbol K-2 1485 14 13 6.92 Comparative steel 613 22 0 3.56 sheet

From Table 8, it can be seen that the part (test piece) made from thesteel sheet of the present invention absorbs greater energy than a partmade of the conventional steel sheet having lower strength, thus showinghigher impact resistance.

TEM photograph of the test piece made in this example is shown asreference. FIG. 8 is a photograph of TEM observation of No. 201 of thepresent invention. From FIG. 8, it can be seen that the high strengththin steel sheet of the present invention contains lath-shaped residualaustenite (black portion of bar shape in FIG. 8) specified in thepresent invention dispersed therein. FIG. 9 is a photograph of TEMobservation of No. 220 of a comparative example. From FIG. 9, it can beseen that the high strength thin steel sheet of No. 220 containsresidual austenite (black portion of somewhat round shape in FIG. 9),although the residual austenite has a block shape that does not satisfythe requirements of the present invention.

Example 3

Sample steels A-3 through Q-3 having the compositions shown in Table 9were melt-refined in vacuum to make test slabs. The slabs were processedin the following procedure (hot rolling→cold rolling→continuousannealing) thereby to obtain hot-rolled steel plates measuring 3.2 mm inthickness. The steel plates were pickled to remove scales from thesurface and then cold rolled so as to reduce the thickness to 1.2 mm.

<Hot rolling> Starting temperature (SRT): Held at a temperature between1150 and 1250° C. for 30 minutes.

Finishing temperature (FDT): 850° C.

Cooling rate: 40° C./s

Winding-up temperature: 550° C.

<Cold rolling> Rolling ratio: 50%

<Continuous annealing> Each steel specimen was kept at a temperature ofA3 point+30° C. for 120 seconds, then rapidly cooled (air cooling) at amean cooling rate of 20° C./s to temperature T0 shown in Table 10, andwas kept at T0 for 240 seconds, followed by air-assisted water coolingto the room temperature.

No. 311 shown in Table 10 was made by heating a cold-rolled steel sheetto 830° C., keeping at this temperature for 5 minutes followed byquenching in water and tempering at 300° C. for 10 minutes, thereby toform a martensite steel as a comparative example of the high-strengthsteel of the prior art. No. 312 was made by heating a cold-rolled steelsheet to 800° C., keeping at this temperature for 120 seconds, coolingat a mean cooling rate of 20° C./s down to 350° C. and keeping at thistemperature for 240 seconds.

JIS No. 5 test pieces were prepared from the steel sheets obtained asdescribed above, and were subjected to stretch forming process withelongation of 3% mimicking the actual manufacturing process. Metalstructures of the test pieces were observed before and after theprocessing, tensile strength (TS) and elongation (total elongation E1)before the processing and hydrogen embrittlement resisting propertyafter the processing were measured by the following procedures.

Observation of Metal Structure

Metal structures of the test pieces were observed before and after theprocessing as follows. A measurement area (about 50 by 50 μm) at anarbitrarily chosen position in a surface parallel to the rolled surfaceat a position of one quarter of the thickness was photographed atmeasuring intervals of 0.1 μm, and area proportions of bainitic ferrite(BF), martensite (M) and residual austenite (residual γ) were measuredby the method described previously. Then similar measurements were madein two fields of view that were arbitrarily selected, and the measuredvalues were averaged. Area proportions of other structures (ferrite,pearlite, etc.) were subtracted from the entire structure.

Mean axis ratio, mean length of minor axes and minimum distance betweenthe residual austenite grains of the steel sheet before and after theprocessing were measured by the method described previously. Test pieceshaving mean axis ratio of 5 or higher were regarded to satisfy therequirements of the present invention (∘), and those having mean axisratio of lower than 5 were regarded to fail to satisfy the requirementsof the present invention (x).

Measurement of tensile strength (TS) and elongation (E1) Tensile testwas conducted on the JIS No. 5 test piece before processing, so as tomeasure the tensile strength (TS) and elongation (E1). Stretching speedof the tensile test was set to 1 mm/sec. Among the steel sheets havingtensile strength of 1180 MPa as measured by the method describedpreviously, those which showed elongation of 10% or more were evaluatedas high in elongation.

Evaluation of Hydrogen Embrittlement Resisting Property

In order to evaluate the hydrogen embrittlement resisting property, flattest piece 1.2 mm in thickness was subjected to slow stretching ratetest (SSRT) with a stretching speed of 1×10⁻⁴/sec, to determine hydrogenembrittlement risk index (%) defined by the equation shown below.Hydrogen embrittlement risk index (%)=100×(1−E1/E0)

E0 represents the elongation before rupture of a steel test piece thatdoes not substantially contain hydrogen, E1 represents the elongationbefore rupture of a steel test piece that has been charged with hydrogenelectrochemically in sulfuric acid. Hydrogen charging was carried out byimmersing the steel test piece in a mixed solution of H₂SO₄ (0.5 mol/L)and KSCN (0.01 mol/L) and supplying constant current (100 A/m²) at roomtemperature.

A steel sheet having hydrogen embrittlement risk index higher than 50%is likely to undergo hydrogen embrittlement during use. In the presentinvention, steel sheets having hydrogen embrittlement risk index nothigher than 50% were evaluated to have high hydrogen embrittlementresisting property.

Results of the test are shown in Table 10. TABLE 9 Steel speciesChemical composition (mass %)* Ac3 Bs Ms Symbol C Si Mn P S Al Cu Ni TiV Nb Ma Others (° C.) (° C.) (° C.) A-3 0.29 1.45 1.50 0.014 0.002 0.043— — — — — — — 847.5 616.7 374.0 B-3 0.40 2.03 2.01 0.012 0.002 0.033 0.30.2 — — — — — 826.4 533.7 301.7 C-3 0.42 1.99 2.53 0.011 0.002 0.033 —0.2 — — — — — 809.4 481.5 275.0 D-3 0.39 2.02 2.51 0.011 0.002 0.030 — —0.05 0.05 — — — 843.1 498.8 293.3 E-3 0.41 1.98 2.48 0.011 0.002 0.031 —— — — 0.05 0.2 — 820.5 479.5 280.6 F-3 0.39 2.00 2.53 0.011 0.002 0.030— — — — — — B: 0.0005 816.4 497.0 292.7 G-3 0.35 1.50 2.50 0.012 0.0020.033 0.3 0.3 0.05 0.05 0.05 0.2 B: 0.0005 824.5 482.8 303.3 H-3 0.551.52 1.53 0.012 0.002 0.033 0.3 0.3 0.05 — 0.05 0.2 Ca: 0.004, 818.8516.1 240.5 Mg: 0.005 I-3 0.40 1.98 2.48 0.011 0.002 0.033 — — — — — —REM: 0.005 816.6 498.8 289.6 J-3 0.7 1.51 1.52 0.011 0.005 0.031 — — — —— — — 782.2 504.2 179.0 K-3 0.39 0.14 1.42 0.012 0.002 0.043 — — — — — —— 772.5 596.9 329.3 L-3 0.29 1.49 1.51 0.011 0.002 0.052 — — — — — — —850.5 615.8 373.7 M-3 0.30 1.49 1.50 0.012 0.002 0.241 0.3 0.2 — — — — —916.2 606.6 365.9 N-3 0.31 1.51 1.52 0.012 0.002 0.387 0.3 0.3 0.05 — —— — 991.5 598.4 358.8 O-3 0.31 1.51 1.49 0.012 0.002 0.740 0.3 0.3 0.05— — — — 1133.6 601.1 359.8 P-3 0.30 1.49 1.50 0.012 0.002 1.02 0.3 0.30.05 — 0.05 0.2 Ca: 0.004, 1252.6 586.3 360.0 Mg: 0.005 Q-3 0.29 1.501.51 0.11 0.002 1.65 — — — — — — — 1490.1 615.8 373.7*The balance consists of iron and inevitable impurities.

TABLE 10 Before processing Minimum Mean distance axis between Mean Steelratio of residual axis Test species To Residual γ residual γ γ grainsratio of BF + M Others TS El No. Symbol ° C. % nm nm residual γ % % MPa% 301 A-3 400 8.5 180 490 ∘ 91.5 0 1221 14 302 B-3 370 11 160 760 ∘ 89 01497 14 303 C-3 370 12 150 710 ∘ 88 0 1495 13 304 D-3 370 11 140 630 ∘89 0 1460 13 305 E-3 370 12 160 710 ∘ 88 0 1489 16 306 F-3 370 11 140630 ∘ 89 0 1488 12 307 G-3 370 10 130 590 ∘ 90 0 1480 14 308 H-3 350 12110 690 ∘ 87 1 1475 14 309 I-3 350 10 100 610 ∘ 89 1 1504 12 310 J-3 3506 1200 700 ∘ 92 2 1422 7 311 K-3 350 <1 — — — 99 <1 1410 5 312 A-3 35015 1500 610 x 20 65 967 15 313 L-3 400 9 190 750 ∘ 90 1 1230 14 314 M-3370 10 160 700 ∘ 90 0 1410 13 315 N-3 370 10 150 620 ∘ 90 0 1471 12 316O-3 370 10 150 620 ∘ 90 0 1471 12 317 P-3 350 11 140 610 ∘ 89 0 1480 11318 Q-3 370 17 1100 1300 x 55 28 1320 14 After processing Minimum Meandistance axis between Mean Hydrogen ratio of residual axis embrittlementTest Residual γ residual γ γ grains ratio of BF + M Others risk indexNo. % nm nm residual γ % % % 301 4 170 510 ∘ 96 0 30 302 5 140 690 ∘ 950 25 303 6 130 650 ∘ 94 0 23 304 4 120 600 ∘ 96 0 20 305 5 150 750 ∘ 950 25 306 4 120 600 ∘ 96 0 23 307 4 120 600 ∘ 96 0 13 308 6 100 690 ∘ 940 15 309 5 90 630 ∘ 95 0 18 310 4 1180 800 ∘ 96 0 60 311 <1 — — — 99 <190 312 <1 1480 710 x 99 <1 85 313 5 170 680 ∘ 95 0 24 314 6 140 680 ∘ 940 22 315 6 140 600 ∘ 94 0 18 316 6 140 600 ∘ 94 0 18 317 7 120 600 ∘ 960 15 318 9 1000 1200 x 91 0 70

The results shown in Tables 9 and 10 can be interpreted as follows(numbers in the following description are test Nos. in Table 10).

Test pieces Nos. 301 through 309 (inventive steel sheets 2) and testpieces Nos. 313 through 317 (inventive steel sheets 1) that satisfy therequirements of the present invention have high strength of 1180 MPa orhigher, and show high hydrogen embrittlement resisting property in harshenvironment after the forming process. They also have high elongationproperty required of the TRIP steel sheet, thus providing steel sheetsbest suited for reinforcement parts of automobiles that are exposed tocorrosive atmosphere.

Test pieces Nos. 310 through 312 and 318 that do not satisfy therequirements of the present invention, in contrast, have the followingdrawbacks.

No. 310 made of steel species J-3 that includes excessive C content hascarbide precipitated and residual austenite of longer mean length ofminor axis, thus resulting poor performance in both workability andhydrogen embrittlement resisting property after processing.

No. 311, martensite steel that is a conventional high strength steelmade of steel species K-3 that includes insufficient Si content, hardlycontains residual austenite and is inferior in hydrogen embrittlementresisting property. It also does not show the elongation propertyrequired of a thin steel sheet.

No. 312, that was made of a steel that has the composition specified inthe present invention but was not manufactured under the recommendedconditions, resulted in the conventional TRIP steel. As a result, theresidual austenite does not have the mean axis ratio and the mean lengthof minor axis specified in the present invention, while the matrix phaseis not formed in binary phase structure of bainitic ferrite andmartensite, thus resulting in low strength and poor hydrogenembrittlement resisting property.

No. 318 includes Al content higher than that specified for the inventivesteel sheet 1. As a result, although the predetermined amount ofresidual austenite is retained, the residual austenite does not have themean axis ratio specified in the present invention, the desired matrixphase is not obtained and inclusions such as AlN are generated thusresulting in poor hydrogen embrittlement resisting property.

Then parts were made by using steel species A-3, G-3 shown in Table 9and comparative steel sheet (590 MPa class high strength steel sheet ofthe prior art). Performance (pressure collapse resistance and impactresistance) of the formed test piece were studied by conducting pressurecollapse test and impact resistance test as follows.

Pressure Collapse Test

Maximum tolerable load was determined similarly to Example 1 by usingsteel species A-3, G-3 shown in Table 9 and the comparative steel sheet.Absorbed energy was determined from the area under the load-deformationcurve. The results are shown in Table 11. TABLE 11 Evaluation of testpiece Steel sheet used Maximum Energy TS EL Residual γ load absorbedSteel species (MPa) (%) (Area %) (kN) (kJ) Symbol A-3 1221 14 8.5 11.30.58 Symbol G-3 1480 14 10 13.8 0.69 Comparative steel 613 22 0 5.7 0.33sheet

From Table 11, it can be seen that the part (test piece) made from thesteel sheet of the present invention has higher load bearing capabilityand absorbs greater energy than a part made of the conventional steelsheet having lower strength, thus showing high pressure collapseresistance.

Impact Resistance Test

The impact resistance test was conducted similarly to Example 1 on thesteel sheets made of steel species A-3, G-3 shown in Table 9 and thecomparative steel sheet. The results are shown in Table 12. TABLE 12Evaluation of test Steel sheet used piece TS EL Residual γ Energyabsorbed Steel species (MPa) (%) (Area %) (kJ) Symbol A-3 1221 14 8.55.72 Symbol G-3 1480 14 10 6.88 Comparative steel 613 22 0 3.56 sheet

From Table 12, it can be seen that the part (test piece) made from thesteel sheet of the present invention absorbs greater energy than a partmade of the conventional steel sheet that has lower strength, thusshowing high impact resistance.

TEM photographs of the test pieces made in this example are shown asreference. FIG. 12 is a photograph of TEM observation (magnificationfactor 15000) of No. 301 of the present invention. FIG. 13 is aphotograph of TEM observation (magnification factor 60,000) of a portionshown in the photograph of FIG. 12. From FIGS. 12, 13, it can be seenthat the high strength thin steel sheet of the present inventioncontains fine residual austenite grains (black portion of bar shape inFIGS. 12, 13) specified in the present invention dispersed therein, andthat the residual austenite has the lath shape specified in the presentinvention. FIG. 14 is a photograph of TEM observation of No. 313 of acomparative example. From FIG. 14, it can be seen that the high strengththin steel sheet of No. 313 contains residual austenite (black portionof somewhat round shape in FIG. 14), although the residual austenite hasa block shape that does not satisfy the requirements of the presentinvention.

1. A high strength thin steel sheet having high hydrogen embrittlementresisting property and high workability, which comprises: C: higher than0.25 up to 0.60%; Si: 1.0 to 3.0%; Mn: 1.0 to 3.5%; P: 0.15% or less; S:0.02% or less; and Al: 1.5% or less (higher than 0%) in terms ofpercentage by weight, with balance of iron and inevitable impurities;wherein the metal structure after stretch forming operation withelongation of 3% comprises: residual austenite; 1% by area or more inproportion to the entire structure; bainitic ferrite and martensite: 80%or more in total; and ferrite and pearlite: 9% or less (may be 0%) intotal, while the mean axis ratio (major axis/minor axis) of saidresidual austenite grains is 5 or higher, and the steel has tensilestrength of 1180 MPa or higher.
 2. The high strength thin steel sheetaccording to claim 1, wherein the metal structure after said stretchforming operation with elongation of 3% further satisfies therequirements that: mean length of minor axes of said residual austenitegrains is 1 μm or less; and minimum distance between said residualaustenite grains is 1 μm or less.
 3. The high strength thin steel sheetaccording to claim 1, wherein 0.5% or less (higher than 0%) by weight ofAl is contained.
 4. The high strength thin steel sheet according toclaim 1, wherein 0.003 to 0.5% of Cu and/or 0.003 to 1.0% of Ni in termsof percentage by weight are further contained.
 5. The high strength thinsteel sheet according to claim 1, wherein 0.003 to 1.0% of Ti and/or Vin terms of percentage by weight are further contained.
 6. The highstrength thin steel sheet according to claim 1, wherein 1.0% or less(higher than 0%) of Mo and 0.1% or less (higher than 0%) of Nb in termsof percentage by weight are further contained.
 7. The high strength thinsteel sheet according to claim 1, wherein 0.0002 to 0.01% of B in termsof percentage by weight is further contained.
 8. The high strength thinsteel sheet according to claim 1, wherein at least one element selectedfrom the group consisting of: 0.0005 to 0.005% of Ca; 0.0005 to 0.01% ofMg; and 0.0005 to 0.01% of REM in terms of percentage by weight isfurther contained.
 9. A high strength thin steel sheet having highhydrogen embrittlement resisting property and high workability, whichcomprises: C: higher than 0.25 up to 0.60%; Si: 1.0 to 3.0%; Mn: 1.0 to3.5%; P: 0.15% or less; S: 0.02% or less; and Al: 1.5% or less (higherthan 0%) in terms of percentage by weight, with balance of iron andinevitable impurities, wherein the metal structure after stretch formingoperation with elongation of 3% comprises: residual austenite; 1% byarea or more in proportion to the entire structure; while the mean axisratio (major axis/minor axis) of said residual austenite grains is 5 orhigher; mean length of minor axes of said residual austenite grains is 1μm or less; and minimum distance between said residual austenite grainsis 1 μm or less; and the steel has tensile strength of 1180 MPa orhigher.
 10. The high strength thin steel sheet according to claim 9,wherein 0.5% or less (higher than 0%) by weight of Al is contained. 11.The high strength thin steel sheet according to claim 9, wherein 0.003to 0.5% of Cu and/or 0.003 to 1.0% of Ni in terms of percentage byweight are further contained.
 12. The high strength thin steel sheetaccording to claim 9, wherein 0.003 to 1.0% of Ti and/or V in terms ofpercentage by weight are further contained.
 13. The high strength thinsteel sheet according to claim 9, wherein 1.0% or less (higher than 0%)of Mo and 0.1% or less (higher than 0%) of Nb in terms of percentage byweight are further contained.
 14. The high strength thin steel sheetaccording to claim 9, wherein 0.0002 to 0.01% of B in terms ofpercentage by weight is further contained.
 15. The high strength thinsteel sheet according to claim 9, wherein at least one element selectedfrom the group consisting of: 0.0005 to 0.005% of Ca; 0.0005 to 0.01% ofMg; and 0.0005 to 0.01% of REM in terms of percentage by weight isfurther contained.